I would go for something like a small group of proper upside-down synos (not the giant version), or possibly something like some porthole catfish.
Other things to consider for a 35 is maybe a small plec like a bristlenose, rubbernose or clown plec?
Just remember to research whatever catfish you are interested thoroughly first as a lot of cats get bigger than you'd expect.
use www.planetcatfish.com for info on ALL catfish!
Ben