All I would put in that is a snail... Most other passive fish are schooling, which you can't do in a 5.5G...
Cories need at least 5 to be happy and a pleco's bioload is way too large for something that small, not to mention they can get over a foot longI've had great luck with two corydoras in a five gallon, or a plecostomus. I've also put in a Blind Cave Tetra with no issue, but ONLY with very calm fish who don't flare.