Blue - Yukino Satsuki, hands down. Aya's great, definetely, but she's no Blue. Yukino, meanwhile, has a character so similar to Blue it's not funny, and she sounds perfect as her.
Yellow - Hirohashi Ryo would prolly be a good one for her.
Gold - If he could get himself to sound young enough for Gold, then I'd say Hoshi Soichiro would be great. Perfect, even. If that was not possible though, a Takeuchi Junko would be fine too.
Silver - Eh. Never been a Paku Romi fan myself, probably never will be, but if you want to stereotype, I guess it'd fit. It'd certainly bring in more fans.
Ruby - Seconding Miyano Mamoru a thousand times over. No, ten thousand.
As for the others... uh, Nakahara Mai might be good for Crystal, but her voice scares me half the time sooo...
Oh and you've gotta consider timeskips and stuff. Would Red, Green, and Blue sound the same in FireRed/Leaf Green as they did in Red/Green/Blue/Yellow, and maybe even Gold/Silver/Crystal to an extent? Red and Green, for instance, might need a bit of a change up (though Blue would probably be okay) seeing as puberty = deeper voices and all. Of course, Naruto's seiyuu is a good example of keeping it up, so.