I would stay with a BN...They max out around 4-5 inches and will not greatly increase to the bioload. I would stay with one in a 55g but if you wanted to you could do more. They may be very aggressive towards each other
well then i guess one is the way to go... i dont want to have to many plecos in there, cause i want to attribute as much of the bioload to the cichlids... you cant ever have enough cichlids
ive had my BN in with cichlids since he was around 2 inches and never had any problems. He would always be hiding until he turned around 4.5 inches and now he is always all over the tank
I keep an albino bristle nose in almost every tank in my growout/breeding rack in my fish room. They are a bit smaller than the regular variety, but do the job well. Almost all the tanks have Malawi cichlids and they don't mess with the plecos at all.
Rubber lips are very hardy and great glass cleaners, but stay small as well.
For a 55, you may want to go with a bit of a bigger pleco. I have a spotted sailfin (not sure of L number) in my 180 with the larger haps/mbuna. He is about 9" long and stands his ground well against some of the predator haps.
Try to avoid the "common" variety as they will get massive and turn into a mobile poop factory.