Heh, I somehow read 550G not 550L, must be getting too used to american measurements, though to be fair I had been up since 3.30am...
Regardless, the fish I sugested will still work, just in smaller numbers. If it were me I'd go for a group of one of the smaller species of Geophagus, some angels or Mesonauta, a school of some sort of tetra (lemons are great), a bunch of corys on the bottom and a couple of small-medium sized plecs. If you want more cichlids you could look at adding something like a single male severum or blue acara, but IMO keep the geos as a group buy avoid breeding pairs of the others if you want to keep the tank peaceful.