I would think that an electric blue jack dempsy would be more suited for a 55 - 75 gallon tank.
I am uncertain as to why others believe that the Paratilapia are slow growing fish? Just because the three Oreochromis species that are aqua cultured grow considerably faster, does not make small spot or large spot slow growers.
Both have reached to 12" + range within two years from fingerlings!
Paratilapia are indeed much more aggressive and less social than the three Oreochromis used in aquaculture. The Nile are ridiculous fast growing (>12" and breeding in less than 9 months from fingerlings) and should be avoided in a 55 due to maximum size. The Mozembique is not a very blue fish.
The Oreochromis Aureus is incredibly cheap, virtually zero co-specific aggression within the species. They are tolerant of less than ideal water conditions. They are SO unobtrusive that you can easily keep half a dozen in the 55. Rule of thumb in Tilapia aquaculture 5 - 10 gallons per pound. You should get 4 pound monsters within a year. unlimited fry & feed the family.
Check this out!
http://brfpc.dssla.servertrust.com/Blue_Tilapia_Fry_Fingerlings_and_Breeder_Sets_s/1841.htm