In my experience, raising juvi Blue Dempseys together is fine, but once they reach sexual maturity males will not appreciate the company of males of their own species. So until you know the gender of your Blue Dempseys you should not plan on them being together for life.
I have kept my large adult male Blue Dempseys with: Oscars, Green Terror (female), Convicts (pair), Firemouth (pair), Elliott's Cichlid (pair), W African Jewels (Pair), Orange Head Geos (pairs), BGK, Severum (male), yong adult Trimac (female), female Dempseys and probably several others that I'm not remembering... (all adults unless noted otherwise)...
Although I do raise my Blue Dempseys with only other Blue Dempseys or other smaller more Docile fish... such as Apistos, Rams, young OH Geos, etc...
I have seen a very wide range of "toughness" in Blue Dempseys and therefore I am not quick to assume they can, or cannot, handle certain tankmates...
My advice to you is to relax, grow your fish out, determine gender, determine your fish's individual aggression levels... Then (and only then) worry about tankmates... for the time being add some dithers to the 55 gal containing 2 Dempseys and enjoy your fish
