no, not impossible. tankmates depend on the individual fish.while some may accept tankmates others will murder all no matter tank size. i wouldnt add another cichlid in a 55, only catfish.
My 8" gold saum got along fine with 2 convicts in a 55g. If they grow up together I think you're fine. I wouldn't want to keep a gold saum in a 55gallon for life though.
OK, thanks for your help. So what I understand is that keeping a gold saum is basically like keeping other ca/sa cichlids (in similar sizes), as for tank size and tankmates?