Agree with the others, have a back up plan in case it doesnt work out. I tried it with Calcifer we had a lot of flaring, nudging and chasing, in the end I did rehome them as he really only tolerated them and is much happier with lamp eyes now
ADFs don't require a lot, get a small 2-3g for themselves (could be a cheap plastic critter keeper) dont need to add substrate (they will find it easier to eat bare bottom), they can survive in range of temperatures (seen them in with goldfish) but ensure have/get a heater as back up and dont let the temps drop too low. They dont like much filtration so cheapwise and good for you make a cheap filter (instructions on here somewhere in the pinned articles) and just add some small ornament/plants, nothing much needed as these little guys dont like too many disturbances and like to have things to hide under for safety. Get imaginative! Hope that helps a little. I was lucky to be able to rehome mine to a friend who dearly wanted them, but I did have a small tank setup ready just in case it went wrong (I also borught one first see how that went and brought a second one a week later as at the time it seemed fine) couple weeks after that was when there was a little unsavoury behaviour and decided my betta was a drama queen
Or instead of having to have a backup plan, just get a bigger tank to begin with, so everyone has space and hidey holes to be able to avoid eachother. The least expensive way to do that is to get a second hand tank from Craigslist/garage sale ect ect.