Great looking setup! I wouldn't add anything, stock looks full to me. But the small catfish like ottos wouldn't be a problem if you did want to add them. Though may eventually be a severum snack.
I understand. Im going to my LFS on Thrusday. Want me to pick you anything up? lol
Rainbows are shoaling fish and should be kept in groups of 5+. 5 could comfortably fit in there.