Hi.
So I know coccidia are in EVERY single chicken poo. Medicated feed works by blocking thiamine to SLOW the growth of the cocci (not to kill it) while chicks develop a LITTLE more immunity... which are NOT a problem unless allowed to bloom out of control... usually in warm/humid environments like that of a brooder.
Now guinea... are they considered a game bird? I know quail and turkey need a higher protein then chick starter offers.
Regarding WHY they feed store told you not to go with medicated... could be a personal preference... or maybe they know something about keets that I don't. Like their protein needs.
What IS GOOD ENOUGH... is MAKING sure your bedding is dry and water is poo free. Not over crowding your brooder. Poo in the water... fastest way to spread coccidiosis.
I don't use medicated starter... because I'm not raising hundreds of birds in poor conditions. I do have Corid on hand... which is the same active ingredient as medicated starter (most), in case I do need to treat... one year I did as there were special pasture conditions.
Now I know the reason they wouldn't suggest medicated starter for duck would be because it doesn't have enough niacin in it, and has nothing to do with the medicated part.
Raw acv.... meh... some people use it. It adds probiotics. If you like to or want to... fine. If your birds on in good condition... it won't make much difference IMO.
Hope your adventure is fantastic!