demon fish--common name of Satanoperca Jurapari...a South American earth-eater cichlid. usually not overly aggressive but prefers a lower ph in the neutral to slightly acidic range. however, tank bred and raised specimens won't be quite as picky about ph. not exactly sure how big these get but have heard 12-14" before so this species will probably get a bit too big for a 55.
severums--Heros Severus...another South American cichlid. These aren't picky about water params. ime. They'll get to about 8" maybe a little bit bigger. These fish are very tall so their overall girth is substantial. I think a pair would do ok in a 55 with some smaller schooling fish in a 55. Or you could do 1 severum and either 1 firemouth or acara.
firemouth---Thorichthys meeki...Central American cichlid that's not overly aggressive but of course will be territorial if a pair forms. As juvies these do very well in groups because they can get very timid when being kept alone. Also these do very well with any type of livebearers except for guppies. These fish are found side by side with platys and other livebearers in the wild. It's important to keep them together though from the time the firemouths are juvies. These'll grow to about 6".
angelius cat---Most likely Synodontus angelicus...an African catfish. I don't know much about syno. catfish but according to
this site they'll grow slightly over 7" and may become territorial with their own species. If that's so you could probably keep up to 2 safely in a 55 with any of these cichlids.
blue Acrara---Aequidens pulcher...mostly peaceful cichlid species that'll most likely grow to around 6". I've never kept them so can't give any information from experience.
Iridesent shark---Pangasius sutchi...these grow in the neighborhood of 3 feet long and are a schooling fish. IMO, these shouldn't even be sold for the home aquaria.
senegal bichir---Polypterus senegalus...Awesome fish. This is one of the smaller Polypterus species usually maxing out around 12-13". These are highly predatory so they will eat small fish. These are not an aggressive fish, though more than one male in the same tank may fight.
Personally, I would do 2 blue acaras, 2 firemouths, 1 or 2 of the angelicus cats, 1 senegalus and about 6-8 platys. if you get the bichir small enough and the platys at a decent size, then I highly doubt the bichir would mess with them. if the platys breed, the fry would certainly be eaten, but the original parent fish shouldn't be bothered.
i have those three living together niceley 
