Here is some good info on tankmates:
"One of the most common questions I address is, "What else can I keep with my lionfish?" The answer is a complicated issue [Figure 19]. Frequently, hobbyists want to keep multiple lionfish in the same aquarium, and although there is no problem housing two or more of these fish together, closely related lionfish will fight. I have found this problem occurring most frequently with the dwarf lionfish, in that Dwarf Fuzzy lionfish will often harass Dwarf Zebra lions. Although these skirmishes are often nothing more than gill cover flaring, head shaking, and chasing, many times it's accompanied by a bite or two. If one lionfish persistently attacks another, they should be separated or the subordinate lionfish will cease feeding and may die. Another point of consideration when keeping both large and dwarf forms of lionfish together is that large lionfish will eat smaller lionfish, and I have personally witnessed this on several occasions.
Special considerations need to be used when selecting tank mates for lionfish. Lionfish will eat any small fish or crustacean that fits into their cavernous mouths. Even though lionfish are venomous, lionfish are not immune to being harassed or even eaten by other fish. Large eels, frogfish and other scorpion fish are all predators of lionfish in the wild. According to Michaels (1998), large angelfish, pufferfish, and triggerfish are also known to harass lionfish. Triggerfish are notorious for nipping off the dorsal spines before killing lionfish. However, I have found the planktonic feeding triggers (Blue cheek, Pink-tailed, Niger) to behave more predictable towards lionfish, and I would consider them compatible tankmates. Because of the small confines of many aquariums, lionfish have been known to impale tankmates with their venomous spines. Envenomation likely occurs for several reasons: the lionfish may be retaliating for a prior attack and intentionally impales the fish, or a tankmate accidentally swims into the venomous spines. When jabbed by a lionfish spine, the victim will usually develop a good size lesion at the point of impalement and the area will become inflamed, reddened, and necrotic. Other signs of the venom on impaled fish are increased breathing rates, distress, color loss and decreased swimming. In many instances, death will occur. According to Michaels (1998), most fish injected with a large dose of lionfish venom (more than what is injected with just one spine) will die with 30 minutes. "