Henlo!
Depending on the feed you use, it likely already contains pre- and probiotcs, so adding more wouldn't really provide additional benefit (just additional cost).
Also ACV has no digestive benefit as the digestive acids in a chicken's stomach are actually MORE acidic than the ACV. (Vinegar also inhibits the absorption of calcium).
The best thing you can do is provide clean, fresh water, in clean containers (and in the winter, make sure their water isn't freezing).
ETA: If the feed you use does NOT contain pre- and probiotics then adding it can be beneficial.