Hi and welcome to the forum!
a 55 is a decent sized tank that you could have plenty of options with when it comes to africans. the important thing to consider is compatibility amongst breeds.
while i have seen it done, i would try to avoid mixing africans with any sa/ca cichlids and this includes the convicts.
Labeotropheus fuelleborni
Labidochromis sp. "Hongi"
Labidochromis caeruleus (Kakusa)
Melanochromis cyaneorhabdos
Melanochromis johannii
Metriaclima callainos (OB)
Metriaclima lombardoi
are a few that you could look into. while each and all vary in their level of aggression, most of this will be determined by the m/f count and the aquascaping that you may have within your tank.
I hope this helps a bit..
Good luck on your hunt for a new breed.
Ps. you may want a few more acei like 3 perhaps..as the school nicely and with the 4 ft length on your tank they would be fun to watch.