What did poultry fanciers do for coccidia before they had amprollium and Sulmet? That's what I would do if developing heritage stock...use a natural way to decrease the overgrowth of coccidia in the coop and soil environment and also in the intestines. There has to be a way to do that or we wouldn't have a chicken one on this Earth.
I do what the broody hen does. I put them on dirt ASAP.
any bird that's ever been on real dirt has been exposed to coccidia, since it lives in the soil nearly everywhere. as such, every adult bird that's been on the ground more than a month has immunity to it, whether they ever showed signs of having it or otherwise.
that immunity is passed
IN THE EGG to the offspring. when they hatch, they have a limited immunity to anything the mother had immunity to. over the next 2-4 weeks of life, this 'hatched' immunity fades, as the chick starts building it's own immune system to whatever it's exposed to. by the time the chick is a month old, the mother's immunity has worn off and he's running on his own now. the mother's immunity will only last so long, and if they're not exposed to the disease-causing organism in the first 2-4 weeks, they will not retain any of that immunity.
so any chicks that are raised off the ground without any contact with adult birds, will have no exposure to anything at all. period. at this point is when most people put their chicks on the ground for the first time. and of course, many (not all but many) will become sick because they have no resistance to anything, and have been exposed to a little bit of everything all at once.
with broodies, these chicks are exposed to everything from day 1. and provided it's mother had also been exposed to the same or similar organisms, the chicks will usually have ample immunity from day 1. That's not always the case, such as with shipped eggs or hatchery birds who's parents may have never put foot on real dirt in their lives or been exposed to anything ever before. but if they were vaccinated, even that limited immunity should pass along to the chick somewhat. so if he's exposed within the first few weeks, then he'll likely have lifetime immunity.
for my own chicks raised indoors in the brooder, I do a couple things. first, I have some 8" square foil pans with just plain dirt from outside in them, and I sprout plain old 'orchard grass' hay mix in them, with 1/2" hardware cloth across the top to keep the chicks from digging up the grass by the roots... I rotate one of these into the pen at least every other day to provide some natural 'dirt' and greens for the chicks (the ones that are taken out are watered and allowed to grow again for a week or more. more seed is added if necessary, but usually once it gets a good root ball going it'll recover quite well.
the other thing I do is mix in a handful or two of shavings from the nest boxes of my free ranging birds in with the new babies every time I clean their box.
the only time I've had a problem with coccidia is with chicks that I've purchased that were already 3-4 weeks old, and most likely kept in 'sterile' brooders...
I will NOT feed medicated feed to the chicks, as it has only minimal amounts of amprollium in it. not enough to actually do anything to coccidia, but still enough to prevent thiamine absorption by the chick in small amounts. and even then, IMO all it's doing is encouraging 'stronger' bugs to propagate... if they can survive minimal thiamine deprivation, then they'll be more likely to take off once the medication is stopped.
I do keep corid on hand for those few chicks i had no control over early on. just as i do keep penicillin and tetracycline on hand for any new birds coming in that may have compromised immunity (because of nutritional or management issues - or lack thereof...) but prefer not to use it if i don't have to...
