When I raised guinea keets last year, I just fed them regular chick starter and they turned out fine. Some people recommend feeding the keets a higher-protein starter like turkey starter, and you can certainly feed them that, but I think chick starter also works.
It should be okay to raise the keets and chicks together, but just make sure that you have plenty of space for them all, of course.
Good luck with your keets, and I hope that this helps.
Edit: This is really just my personal experience; I'm not an expert on how to raise guinea fowl, and chick starter was all we had. The guineas turned out fine, though, so maybe it varies from keet to keet. Sorry if this post was misleading.