Yellow labs are great! And in a 55g I suggest a group of 6. Most other tankmates you will find for them are a bit more aggressive, so I'd have more of the labs.
In my Malawi tank, I have yellow labs along with Pseudotropheus socolofi albinos (aka Snow white socolofis), Cynotilapia afra cobue, and some Eureka peacocks (A. jacobfreibergi).
Other possibilities are Rusty Cichlids, maybe Red Zebras, though sometimes these guys can be really aggressive (depends on the individual fish I guess), and other peacocks, since they share a similar diet. I'd stay away from the more aggressive Mbuna, just because Yellow labs are not that aggressive.