I think we have to take everything within the context of each group.
The "yonko" are not a gruop, it is merely a way to refer to to 4 pirate crews with extreme power who work independently from each other. The shichibukai are not a group either, they are merely 7 independent folks who work for the world government. In this regard, it is not appropriate to compare the yonko and shichibukai as a whole to other forces.
Based on what we saw I would think that at least each individual yonko has in some form military strength comparable to the marines. Luckily the yonko are also each others enemies so the marines don't have to deal with them at one. WB had the larger crew but I kinda doubt that means much. his commanders were his main crewmembers, the rest were either normal people or rookies not worth mentioning. The reason WB was so hard to fight against was that along with his crew he also had 50 new world captains behind him and their crews of which each captain and main crewmembers probably had a decent degree of strength. The combination of his 14 commanders and 50 new world captains allowed WB to fight evenly with the admirals, VAs and elite marines that the world government had. As far as infantry goes, the WB allies probably could match the world governments military force but I would still think that the vast mayority of the army's power was large concentrated in the commanders and captains just like the marines had the vast majority of their power concentrated on their admirals, VAs and other elite marines.
Now, are these numbers hard to match for other yonko? A yonko having 10 strong crewmembers is not that out there for the most part and gathering 50 allies is not that far out there either. Of course, for many strong people gathering these number of strong fighters would probably be a challenge in itself.
The revolutionary army would appear to fit a similar condition. They have dragon, their fearless leader, and their commanders who based on the guys we have seen would appear to be strong as hell (kuma, ivankov and inazuma).
About the balance of the world I have something to say. Based on what we have seen throughout the manga, you can get military strength from numbers but ultimately you can only do so much with that. The world government and the WB pirates had numbers in the thousands and yet what made the biggest difference in the war was not military strength (it did help though) but rather how strong the strong individuals on their side were. Even large scale military tactics used during the war had to necessarily be backed up by someone with actual strength, the masses on their own could not hope to oppose strong individuals. If we consider this, the importance of the shichibukai in the balance of the world becomes evident. If the 7 shichibukai fight the WB pirates or the world government as a whole I would think they'd get butchered to say the least. On the other hand, the shichibukai are not expected to take on the entire military and individual might of those forces. The strength of the world government and the individual yonko lies in the few members they have who have a degree of strength. In this regard, losing a measly one person of strength can have a huge impact.
The marines have
1 fleet admiral
3 admirals
20 VA (we have seen around that many at least)
randome marines with a degree of strength (smoker for example although he is not quite strong enough to make a difference in the grand scheme of things)
militia (worthless fodder incapable of doing anything on their own)
WB had:
WB
14 commanders
50 NW captain allies
Random strong members who are not capable of making a difference in the grand scheme of things
Militia
This is where the shichibukai come in. They as a group are probably not a match for the sheer number of strong enemies that large organizations have (7 vs 20, 30 or 60 right?) but in turn the shichibukai add up to the forces the marines have. the situation was not that the marines were equal to the 4 combined yonko hence they were in balance with them but rather that the marines and each individual yonko have comparable military forces and as a result a delicate balance was reached thanks to the individual strong members they have.
its not like this: marines= shichibukai = yonko
but rather : marines = kaido pirates = red hair pirates = WB pirates = big mom pirates
This is where the shichibukai come in. Since there is this rather delicate balance of power maintained not by the strength of their militia but rather a select group of strong individuals we get that a mere 1 extra member can make a HUGE, OVERWHELMING, and ENORMOUS difference in how these gargantuan forces interact. In this regard perhaps the scenario which we have is perhaps more along the lines of these:
marines + shihcibukai = kaido pirates = red hair pirates = WB pirates = big mom pirates
Of course even that is a rather simplistic view of the whole thing, for the most part this would work only in an scenario where the forces collide against each other with their full might as was the case in the war. In more general situations the actual strong people of each force would actually be scattered around fulfilling a number of roles which would include leading the militia which in turn would add up to the chaos. You can't have that many strong people everywhere either meaning that if things go wrong one of the players here could go out of his way to send an actual strong fighter and have a militia with not very strong people obliterated without the chance to fight back.
Heck, even what is happening with hodi's army shows the importance of lone strong people. Hodi had a fantastically well organized army. he had each group fulfill specific fighting duties such as poisoning, close combat and so on...Heck, his army was in itself larger than the marines and WB's force during the war. What went wrong? He did not have actual strong people backing them up. If hodi actually had strong people backing his army then the strawhats would not be capable of freely mowing through them. Heck, if there was someone strong backing the armies up then perhaps they could actually make a difference when dealing with the strawhats as they could attack along with whoever is leading them.
!! Who luffy trust the most?? who fought as equal with whitebeard?? who put an end to marinford war?? Who is even feared by mihawk?? wich crew is feared by the admirals?? Blackbeard had to retreat before someone who?? so much question and one answer....!!!! as for the rest its useless to determine which group is stronger as the new worlds as yet to begin ( but hypothesis: shanks best crew of all time; blackbeard crew boss final in front of one piece; one problem is the last yonkou but the Revolution army will probably take care of that... then smoker will be troublesome but coby will reappear!!! ahahah I can be all wrong but...
