Yeah plecos are just poop machines that's for sure lol. I'm a big pleco guy myself, always liked the fancy ones like L200 and some of the others like snowball and even ABNP's, but over the years I have cut down to just one ABNP and one King Tiger pleco mainly just because they create sooooo much waste in the tank.
Another thing you could do with the Y.labs and 55g is a small group of peacocks. Peacock cichlids are for the most part very relaxed compared to mbuna and others in the African family. And Y.labs usually get along with peacocks. So you could look for 4-6 nice peacocks like maybe a flametail, sunshine, german red etc etc. They would max out at around 5" and be okay in a 55 with your labs. Defiantly don't put them in if you keep the Kenyi though. I know its hard to get rid of a fish you already have because you hate to let them go, but I REALLY think you'll be best trading in the Kenyi. He's only gonna get more murderous as he grows. They are a mean little mbuna. Mine which is only 4" even chases some of my 8" predator haps, and thats in a 6ft 240g tank.
Far as sand, its actually easy to take care of a sand tank. IMo easier than gravel because you just hover the hose about 1" above the sand and it sucks up the waste. Where as waste tends to fall down in between gravel. Plus most cichlids are sand sifters and they will enjoy having the sand to dig and sift through.