In a 55 I would suggest some of the smaller types; jewels, rainbows, keyholes, convicts, firemouths etc.
I have a 55 set up for my firemouths with pea gravel, DW, lots of live plants and terra cotta flower pots (they love them for spawning.)
With cichlids you can do anything, but there is one constant, they LOVE caves. They are very territorial, caves help them mark their turf, give them a retreat and give them a spawning site.
Some cichlids dig more than others. My JDs never touch a thing yet my firemouths and my mbuna dig like they're looking for gold. So you have to take that into consideration. Also some prefer a higher pH like mbuna, but they can adjust to any stable pH. Then think of feeding, SA/NW cichlids like JDs, FMs, keyholes etc need high protein diets, mbuna need low protein or they will bloat.
Your best bet, see something you like, research the ADULT size the fish will grow to, check out the feeding habits, natural area it would come from, if it needs anything special, then go from there.