There are over 3000 different species of cichlids, some are dwarf, some get 3ft in length, so asking a general question about them is not really answerable.
Some are sub tropical and prefer cool water, some need brackish.
As Rocksor said, you should be looking at South American dwarf cichlids, (any of the types he listed). And to help narrow that down a bit, with your soft water, that means cichlids from east of the Andes and those of the Amazon basin (Peru, Venezuela, Columbia, Brazil etc). Thee are also some African soft water dwarks, such as the Pelvicachromis.
And nothing that gets larger than 5.5" for a 55 gal, or nothing over 7.5", for 75 gal tank.
Avoid, Oscars, Chocolates, large pike cichlids, and large Geophagines, even though they are from that area and water type, they get too large for your tanks, and......the cichlids from west of the Andes are more hard water species, these like green terrors, and festae, should also be avoided with your water type.
The southern South Americans like the Geophagus braziliensus group are also hard water type and get too large for those tanks.
The Gymnogeophagines also require different conditions like cool downs, which the cichlids recommended above, do not.