Well clown loaches get to 12" but they get there slowly. IMO, they shouldn't be in a 55 gallon but many people do it; some with plans to upgrade as the loaches grow. They aren't aggressive - I don't know why memnon thinks so. They are also very social and should be in groups of 3 or more of their own kind. Obviously though, with the size they get to, you can't keep many. They do well with most any fish that they won't be able to eat as they get larger. They are also quite sensitive to water quality and prone to getting ich.
I'd say cichlids would be a great idea for your tank.
If it's going to be planted though, you may have to go with smaller neotropical and dwarf cichlids. Things like rams or apistos. If you do go with such cichlids, you'll be able to breed many of them if you have a lower pH and softer water. They aren't as aggressive as many other cichlids and many can be kept in a community tank with fish like tetras, livebearers and gouramies - so you get the idea. You'll need to research whichever species you preffer as their temperament and prefferences do vary. I like bolivian rams myself - they like harder water than german blue/gold rams, are generaly hardier and are somewhat more aggressive. They also aren't as expensive as many apistos tend to be.
If you would like something larger and maybe a little more aggressive, I'd research mbuna and other african cichlids and see what you like. Fish like yellow labs, rusty cichlids, p. socolofi and p. saulosi are very nice to start with. The problem with these is the hard water and high pH they need - and most will destroy your plants...
Another one to consider might be pulcher/kribs. Very adaptable, not much larger than most dwarf cichlids and though often more aggressive, can be kept in a planted tank without as many problems. They are also quite easy to breed.
Ofcourse there are other cichlids like those larger and more aggressive american cichlids like convicts and other africans other than those I mentioned from various rift lakes etc. And of course there are discus and angelfish