Hey AJ! I'll give you a brief run down on what I read. The can be kept in pairs or schools. However, they can reach a large size....some 9-10 inches, depending on species. Mojoreef has 3 Henis in his 1000gallon reef. I'm not sure how territorial they would get if confined in a system smaller than 100 gallons or so.
Ok - just pulled up this page, if you haven't read through this, maybe it will help...
Heniochus spp.