Yama obviously had the time and experience to pick the four captains he chose to send to Hueco Mundo. It seems like some of them may have asked to go/Yama took in their personal issues into the mix. Based on what you know about the situation in Fake karakura Town, and the situation in Hueco Mundo, whom would you have sent to give the greatest benefit to Soul Society? You can send four captains to Hueco Mundo.
1) Mayuri Kurotsuchi. I think Mayuri was definetly a great bet to send to Hueco Mundo. The pure fact that he had intel on his enemy before he went already makes him a good choice. Mayuri isn't an uber captain, so he's not a top choice to have on the front lines. And then, if he did happen to stumble upon any worthwhile discoveries, he'd be able to relay this/study this, and bring it back to Soul Society.
2) Hitsugaya. Yama HOPEFULLY knew Aizen would bring only the best espada with him to fake karakura town. Hitsugaya needs more experience before he can go toe to toe with the best, his bankai is immature, and most of his knowledge is probably based off books and not battle experience. Sure he's a genius, but he is far from the top (give him 100 years, and he'll be a Yama though). Keep matsumoto in real world though.
3) Kenpachi. I do agree with sending Kenpachi too, for one reason. He IS a loose cannon, he rank off from a captains meeting when ichigo/co were breaking into Soul Society, imagine what he would do if there were enemies thousands of times more powerful than they were at a time. Ken would go nuts. Regardless if he is insanely strong, maybe strong enough to take on the top 3, he can't follow orders to be effective.
4) Soi Fon. Erm, pretty much same deal. Her power level is too low to be able to contend with the top espada. She'd fair well against the lower echelons of them. I doubt she's fast enough to get close enough two times to the top espada, she had alot of trouble with Barragan, and her bankai seems like it's only use IS it's missile, which seemingly drains her after. Keep Omeada in real world.
And then also send the 4th division vice captain.
Honestly, why did Yama think it was a good idea to send both of their main healers, when jes...erm, Orihime was in Hueco Mundo. In addition, Unohana likely makes a strong offensive force.
And Byakuya is within the upper levels of the captains, mid-level at worst.
before i answer id just like to sa a couple of things.
Firstly you cant send Hitsugaya to HM as he needs water and there is none there. He's a good bet for the real world.
Secondly Soi Fon only couldnt get next to Barragan because of his special ability-he's the worst person for her to fight. Shed be able to get close to anyone else besides Zomari-dont kid yourself into thinking the espada are faster than the fastest captain.
Personally I'd send:
1. Zaraki Kenpachi-He's a beast and hes likely to rampage through several lower level espada but without bankai he isnt anywhere near the level of the top espada or captains. I think he's awesome but he;s just not as strong as say Byakuya.
2. Kommamura-again he's powerful, not enough to take out the top espada (unless he shows us more). He'd be enough for a rescue mission and to wreak havoc in Las Noches but not someone i'd want next to me when Aizen and the top 3 come-I'd want Byakuya instead for example.
3. Kidou Corps VC-I think one like that could offer great fire support
4. Kurotschi Mayuri-what you said is exactly right I think.
5. Hannatarou Yamada and Kotetsu Isane to act as healers-maybe even 4th div 3rd seat.
As for sending more captains, I hear what you're saying and I would do that if that mission was a priority for me but for me it wouldnt come close to being a priority. I picked the team with the idea of get in, do damage, get Ichigo and co and bug out. I'd much rather have Byakuya's ultimate badassery next to me when Aizen and his top espada come a knockin'. Id like to have soi fon as well but she could be helpful for an extraction mission. Beside I imagine she could probably terminate 2-3 of the lowest level espada in shikai lol
Well... in hindsight of the battles, Kubo did match up the captains against perfect opponents in HM.
Evil genius vs. even better evil genius.
Guy who can control things vs. guy who has 1 million things to control and a bakudou to block it.
Crazy strong reiatsu physical fighter guy vs. even crazier and even stronger reiatsu physical fighter guy.
And of course the reinforcements from 4th squad were nice, since Orihime was being held captive by Ulquiorra and all.
So the only possibility is Yammi, but we don't know his fighting style in resurreccion. If it is the same as non resureccion (slow, powerful, dumb), then Ken would be a matchup for him as well, except Ken still knows battle tactics, he just purposely decides to ignore them (he knew that kendo would increase his power, but only used it as last resort, same with bells on his head decreasing power, and eyepatch). However, it's possible that Kubo will match up Unohana perfectly with Yammi in the end, so we don't know.
However, without the hindsight, and with the limited info on espada they had from before, I do think that Yama made good choices.
At the time, they did not know AIzen's move, yes, they predicted an atttack on KT, and they set up FKT and all, but they were not sure when AIzen would attack, or at least Ichigo's reaction suggested that they didn't know when Aizen would attack. So sending in mostly mid-tier captains+Unohana and 4th squad for support (and hopefully more from Unohana) seemed like the right decision. Since sending in lower tier captains might might end badly, at that time they didn't know exactly how strong the espada were. And sending only strong captains would leave FKT/SS defenseless against Aizen and the espadas.
All in all, I think Yama made the right choice from the info he had, and in hindsight, with Kubos incredible Captain-Espada matching abilities, it was completely successful save for Yammi, who we are still waiting for.
I think we have to consider that Yama did not know they would be stuck, so he would send a small but very skilled group.
Ken and Unohana seem to be a nobrainer. He had to assume that they needed healery and Ken should be able to easily take care of any hollows, menos, adjucas or whatever they encounter. I also think he would have asked to go. Something about being in the middle of the enemy territory seems like his thing.
Mauri knows what he is doing and should anything go wrong, he would have the best chance of fixing it.
The last I think was reserved for a flexible character. Hitsu, byakuya (gin if he was a goody) would be good choices. They didn't know what was going to happen so someone adaptable is good. Byakuya probably just has more experience.
If I were the old man and weren't a dimwit like him I wouldn't have sent anyone.
opponentsvictims in FKT just as easily and been around for seconds.
Well, for the halibel being useless in Hm issue, I don't think she ever said she needed the surrounding moisture to make her water attacks, so presumably she might be able to just make water out of just her reiatsu or something like that. Hitsu on the other hand... is not so fortunate, at least with the reformation of his bankai.
Maybe it is more like an extra thing his bankai can do, should he be near water. The reforging of his bankai if he looses a fight and it shatters. The ice prison thing he did to luppi and the snow rain thingy.
Most shinigami are self contained fighting machines. It seems disappointing if Hitsu wasn't
What he should get credit for is taking out 2 espada and one Privaron. That was awesome and a great help to SS but he hadnt done that at the time Yama decided to risk 4 people who should be worth more than him to go save him. TBH I think what forced Yama's hand was more that whatever he did Byakuya would end up rushing to HM to save Rukia and Zaraki would go to murder Arrancar and he would probably be powerless to stop them, so why not turn it into an official serach and rescue and get the credit lol
Meh, Yama probably sent 4 captains because Orihime was in HM and if Yama had any idea of her ability he probably concluded that Aizen would use Orihime to reject the event of Urahara sealing the Hougyoku's full power; thus returning it to full power.
Given the information available at the time Yama probably realised the most pressing issue was to get her back.
1. I do believe Mayuri was a good choice. Research, get new intel/etc. He probably wanted to go too given he's a scientist. If an astronomer was given a chance to go study stars/moons/planets on the actual planets, they'd probably go there right away.
2. Kenpachi was a good choice too. He probably wanted to go too. Afterall, he loves fighting but in SS he doesn't really have many people to fight. And I'm sure any hollows that go to SS on the odd occasion probably can be handled by ranked subordinates.
3. Byakuya probably asked to go due to Rukia. Given his experience I assume Yama thought it alright for him to go.
4. Unohana was the wrong choice. Should have just sent her VC, or just Hanatarou. Unohana should have either stayed in SS or more likely be in Fake KT to heal the wounded.
Of course, Yama wouldn't have sent Unohana if he was not in a manga. Unfortunately this is a manga and if Unohana was in FKT than any damage that the good guys got wouldn't have been as shocking because Unohana would have easily been able to heal them.