Can't comment on garlic except to say that I don't use it, nor do I feed medicated feed and I've also never had coccidiosis with chicks raised on the ground... The only time I see coccidiosis is when I raise them in brooders, and that includes brand new brooders that get stripped and cleaned...
I've taken a few birds with lice, weighed them and then given twice the recommended dose. Checked them daily for several days before finally dusting. Anyone that claims it works on poultry lice has not re-check their birds, IMO.
I'm sure one of my faverolles has them again, maybe I should...
I know for a fact that it doesn't work on feather lice even at twice the recommended amount. I had vey high hopes when I bought it, lol. Maybe I could use it on the horses instead, but need to check with my vet first... Remember hearing there was a reason not to, just can't remember what it was...