I think a flame would do well in a 55 as long as it's well established(at least 3 or 4 mos. I've heard some say even longer). We were conned into buying one right after our tank finished cycling, BAD IDEA! It got sick within hours(ick, then popeye), and almost didn't make it. It's been about a month since it appeared to recover(spots gone & eye is completely normal), and the flame is doing great! We feed it lots of seaweed selects soaked in zoe, along with formula one & two and flake, and we also bought some rock with a good growth of green algae on it from LFS tank for the flame to graze on. Don't put it in too soon, or you WILL have problems.
HTH,
B