Even then if it was just those two or those three then I think they would beat the kages. None of the kages have much of a defense against genjutsu and I sure as shit know that only Raikage has a chance against Pain.
Itachi is faster than all of the kages except A so he could just take them out with Amaterasu or Tsukuyomi or anything else really, and if they are on the offensive then he has alot of options plus Susanoo which when it is up makes him invincible.
As long as A is fighting Pain then the kages don't really stand a chance. The only problem would be is if all the kages focused on Pain, because he can't use shinra tensei fast enough to block all their attacks. But if Itachi can fight any amount of the other kages except A, (not that I think Itachi would lose but he would have to devote all of his attention to A to beat him due to his speed), then Pain can take down Raikage and the rest would crumble.
Onoki's dust technique would be useless against Pain who can repel it or just suck it in with his fat Path who sucks in chakra, similar to Samehada but we haven't seen a limit to fat Path's chakra stealing abilities, he can suck in ninjutsu so Pain has two defenses against that. And Itachi can always put up Susanoo to to block it with the Yata mirror and hit Onoki with the Totsuka sword while he was doing his Dust attack.
Mei - same as Onoki's weaknesses against Pain and Itachi except she can't fly.
Gaara - He stays stationary when he fights so Itachi could seal him or put him in Tsukuyomi really easily, and Pain could use shinra tensei to bring him towards him to just stab him or drain his chakra.
Tsunade - Gets bossed by Itachi's genjutsu or Amaterasu or really any of Itachi's attacks. Pain also just demolishes her, I shouldn't have to explain why.
A - The only real threat to Itachi or Pain, his only chance against Pain is to take out the other Paths while the other Kages distract Deva Path. If that happens he is fast enough to get in there as long as Pain doesn't use shinra tensei to suck him in and stab him instead of repelling him. Itachi would have problems with A but he can just put him in a genjutsu, it doesn't have to be Tsukuyomi. And Itachi's Susanoo won't break like Sasugay's.
By the way Algalon, I'm just posting my opinion I don't want to make it seem like I am attacking you for even thinking that they had a chance. I also think it would be a better fight if it were just those two against the other kages.