Really? When? I don't remember that. I think they are special Regulars but Regulars nonetheless. When people talk about Irregulars it's always Enryu and Urek. Disliked because they are troublesome in the Tower. If Zahard was an Irregular I don't think they would be disliked.
If we strictly follow the definition that an Irregular is someone who entered in the Tower of his own will yes they are but I don't think it's that simple. In the story they are never really treat
The 10 Great Warriors are - in the truest sense of the word - Irregulars like Urek, Enryu, Phantaminum and Zahard himself. They are by no means special Regulars. The reason why they are not thrown in the same box as Urek or Enryu is because they are the founders of the current empire. The whole Regular system didn’t even exist before said foundation, so the terms "Regular" and "Irregular" were non-existent and kinda don't apply to them.
Furthermore, the reason why Irregulars are disliked originates solely from Phantaminum’s assault on the "Zahard family". Since this attack, Irregulars are labeled as something like "terrorists". And that makes sense, since Irregulars are the only legit threats to the Zahard empire and Zahard himself. The reason why the 10 Great Warriors seem to be excluded here is because they are - seen as - loyal followers of Zahard, simple as that.
Theoretically, the 10 Great Warriors could do the same shit as any other Irregular (killing Guardians? Violating contracts? Killing Zahard?), given they are "powerful" enough. But it seems that’s not the case. Arie Hon only follows Zahard, because he lost to him 10 times out of 10 fights. And Hon is rumored to be the strongest fighter of the 10 Great Warriors, which means there is quite a huge gap between them and Zahard.
Didn't SIU say phantaminum is not going to be featured in TOG? The guy is basically an omnipotent god anyways, including him in the ranking altogether is pointless. As the situation stands a billion billion billion mazinos would stand literally 0% chance of defeating phantaminum.
SIU stated that he doesn't know if Phantaminum will appear or not, since his character goes beyond the "Tower of God universe". Truth to be told, it wouldn't make any sense for Phantaminum to appear in Tower of God. He is - as you said - the god of the ToG story and one of the top 5 Axis in the Talse Uzer Story, a being who is stated to be powerful enough to destroy the ToG story
instantly. As the author of the ToG story, he doesn't need to appear or interact with anyone. He can do as he pleases.