lionfish can eat food that is equal to the size of their bodies, almost. often, dwarf lionfish (fu manchu lionfish) have a hard time eating at the start. They are pretty, but hide a lot. Most lionfish hang out under the rockwork and hunt at night, except for the volitan, which gets much larger than a foot long. I think you could do a red dwarf in your 29 gallon, but don't plan on seeing it much and it may cause a problem with feeding.
-Doc