What do you mean by "Part time" fish? Are you going to get rid of them when they outgrow the 55 or do you have another large tank to put them in? If you're planning on keeping them and putting them in a large tank later, try to find cichlids that will live comfortably in your biggest tank. A Jack Dempsey or Texas could live in there for a while. You just don't have room to cram a bunch of aggressive cichlids in there safely. You could easily stock some smaller cichlids in the 55 that will live for life in that size tank.