When I was at the LFS to get the Peruvian Otos last Wednesday, I stumbled into Gymnocorymbus socolofi. They were large enough, all 18 together were as big as 1 of the 6 Metynnis that desperately tried to get my attention, they don't eat plants and they were very affordable.
Once in my tank some Ageneiosus tried to take a gulp, but once they found out they were too large they left the Characins in piece. And after that first night - in which I kept one light on for safety reasons - they feel very much at ease.
This is what they look like:

