my standard bristlenose catifsh is 6" long, clearly too big for a 5G
my albino bristlnose has stayed at 3", so I could see how that could work, but I don't recommend it ...
my bulldogs / rubber plecos (chaetostoma sp) have all stayed around 2-2.5" ... I keep one in a 12G nano-tank, and that's about as small as I would go ...
nope, for a 5G I can only truly recommend otocinclus cats ... which is not to say that you couldn't make it work with something else (clown, rubber, albino BN), just that I would not recommend it ...