If its for life, i thing a GT would get too big even. Sure it probably wouldnt get over 12", but if its mean and a glass banger then it will probably hurt itself spazzing out at you. I personally couldnt imagine my large GT in a 55. I have mine in a 72 and am already thinking about a bigger tank for it.
Really though, a 75 gallon tank would be minimum for alot of the fish being mentioned here IMO. If you do go the red devil/GT/trimac route, at least get a female or a bigger tank.
I think your best bet with a 55 is firemouths or salvinis if you want males. I think Rainbow cichlids are also very cool, saw some the other day at an LFS and they were at the glass and seemed very interactive.