The general rule when buying a lionfish is that it will try to eat any fish or moving invertebrate which fits into it's mouth. Of course as a lionfish grows it's mouth gets bigger and so tank-mates are limited to rather large fish. There are exceptional lionfish, individuals can grow up with smaller fish, actually an appropriate analogy might be the lion-king with Timone and Pumba, and consider them "friends not food". Realize that at any moment these lionfish have the potential to become killing machines, which has happened to more than a few awestruck aquarists. This article observed wild invasive Caribbean lionfish continuing to predate on small fish in an area when other predators had moved on. It postulates that lions are such efficient hunters and that prey may not recognize them as dangerous, that their persistence pays off; they stick around until they eat every last small fish. The scientists think that selective pressures may choose for fish that recognize lionfish and learn to avoid them. I have personally seen many examples in aquariums where this is the case; a pseudochromis or goby having learned to avoid the lionfish, and maybe this provides a glimmer of hope for the Caribbean reef fish vs the "terminators" of the sea.