Bala's get to a nice "decent size" of 8+ inches. With a maximum around 14-16 inches, if they have enough room. The common pleco will max out around 18 inches, though the bristle should stay around 5 inches. You've already got quite a few 'heavy hitters' in this tank for a 55 gallon.
Cichlids are best kept only with other cichlids, though some combinations are possible. They're also rather aggressive (Most varieties) and would probably harass your other fish.
The tank seems pretty full, but if you want to know a fish that grows to a "decent" size and is known to live with what you've got, tinfoil barbs come to mind. Beware though, they're another 6+ inch fish.
There's also silver dollars, an 8 incher, and pink kissing gourami's, which can hit 12 inches.