Best case scenario, they don't pick up enough coccidiosis to affect them, so nothing happens. Worst case scenario, you might need to treat with Corid. I'd recommend having some on hand, or figuring out where you can get it quickly if needed.
As far as the feed, I'd just pick one feed or the other (do not mix as that dilutes down the amount of coccidiostat). And just to clarify, if you are in the US and planning to sell eggs as organic, the chicks must be on organic feed from the start (in addition to being certified and all that). If these are just for eggs for your own use and "organic enough" is fine with you, then you can use both conventional and organic if that fits your own needs.