Honestly might be better to put in dither fish he might not think of as threats to his territory. A 55g isen't that big, even for a salvi. My pair claimed a side for them selves, and took out the 3rd before I could get him out. This was in a 125g with a 13" Midas keeping the peace. I have platties in the tank and they might chase them away, but can't really focus on one since I have a good schoal of them. Perhaps trying the same with some colorful dither now before s/he gets too large would be a good idea. I would forget putting other cichlids that it will have to be territorial with, because ones that get too large for a salvini t handle would be too big for your 55g. They are pretty much a mini Umbee. Colorful, full of teeth, and once they set there sights and know they got another fish's number they are relentless.