So what your saying is =will my lionfish stop do what nature has asked it to do for the last millions years? Ah----No.
Lionfish will eat whatever it can fit into its mouth, plain and simple. Small fish, medium sized fish, shrimp, crabs, all food. Big lionfish will also eat smaller lionfish so venom is NOT a deterant.
W/ that said get larger fish, any fish slightly larger than 1/2-2/3 the body size of the lion is often (but not always) safe). I find tangs, groupers, large wrasses (like harlequins), work well.
Lastly, remeber that many species of lionfish get big, so if you have small volitans now it will be a big volitans in 6-8month and then all your fish will be food, if its a dwarf lion species then you might have a slight chance (bar goby nope, small clown-nope)
Just remember small fish=food
frank