Congrats on the good deal.
The ROA uses .457" round balls. You can range from 20 gr to 45+ grains of powder, based on your needs for accuracy, recoil -- and smoke! I've tried most of the conicals in mine and always found the RB the best. By the time you ram it and the powder ignites, it's not really "round" anymore anyhow. Mine shoots best with Pyrodex, and I can shoot 12+ cylinders without any binding of the cylinder pin. With real black powder, 1 - 2 cylinders is all I get before she needs to be stripped and cleaned.
By the way, the RB -- IF placed right -- will kill a bore "eventually", but it wouldn't be a quick stop like you probably want with those nasty critters. I wouldn't do it unless it was all I had and it was charging me. If it was running away, I'd probably let it go rather than risk wounding it -- or worse yet -- making 'em mad!
As for the drop-in centerfire cylinder, due to the bore size, I believe that .45 Colt is the only caliber. I think an outfit named "RD" makes them? Do a search for Old Army on this site and you can learn more about these outstanding firearms.
Tight groups.
Old No7