Am going to offer some info here.
First of all you can keep about 12 fish plus a BN pleco or Synodontis Multipunctus as a bottom feeder in a 55g tank.
Some of the more easier to keep are:
Metriaclima estherae (Red Zebra)
Pseudotropheus saulosi (Best in a species only tank)
Pseudotropheus sp. "Acei" (get around 5-6")
Labidochromis caeruleus (yellow labs)
Iodotropheus sprengerae (Rusty Cichlid)
You would be able to house 3 species of the above in a group of 1 male to 3 female ratio. Mbuna (Lake Malawi rock dwellers) require a male to female ratio in order for the male to not harass a single female and spread the aggression.
Bear in mind also from the above list Red Zebra will crossbreed with yellow labs if you have them both, so you would not be able to sell, giveaway or trade and of the fry as this will taint the bloodlines with hybrids.
If you wanted a more aggressive setup then look at (10 fish):-
The Johanni and Elongatus would need a 1 male to 5 female ratio, any less will result in dead females. Elongatus would be better in a group of 1 male to 8-10 female, they are extremely aggressive.
In terms of filtration at MINIMUM you would need 10 times the tank turnover an hour, 15 times being a better rate to aim for. It would be highly beneficial to get 2 x Aquaclear 110's for this tank. Cichlids are messy fish. UnderGravel filters are nearly useless for most cichlids as they will dig the substrate and expose them, they also because they dig clog very easy and if not properly maintained are a nitrate factory.
If you wanted a more peaceful tank with color then look at an all male hap or peacock tank, this would require another tank to be cycled ready to go in case you have to remove fish. You can add females but in a 55g only one and about 9 fish in total in the tank.
You do not need to get a specialist cichlid substrate, play sand from most hardware stores is more than suitable. Plants are very hit and miss with cichlids but some of the more hardy ones are Jave Fern, Anubias and Vallesneria, anything else and they will chew at it to see if they like it, if they do, it will be gone quickly.