As previously mentioned, a BristleNose plec would be a great addition to an aquarium your size. They only grow to 6" long, so easily enough space for it. Also they're quite widely available, seen lots over the last 6 months in various fish stores, nearly all very small - an inch or so. I got mine at this size, and the rate she's growing at is definately noticeable. I also think she's a great addition to the tank, full of personality, and very active during the day for a plec.
edit: should also say that yes, they are very messy. With my BN and an army of shrimps in the tank, there's always a lot to remove from the substrate surface during water changes, they always seem to be 'pooping'!