Personally, if I had a 55, I'd be getting a couple of the smaller 6-7" Geophagus species like Pindare/Araguari/Tapajos, a pair of German Blue Rams, a solid school of 12-18 Tetra of some variety (I'd use Serpae or Bleeding Heart), and 3-6 Corydoras and 1-2 Bristlenose.