I think pleco's (they're just tough mofo's that can hold their own) and raphael (or talking) catfish (very secretive, and only come out at night, when the p's are sleeping) are the ones that have most chance to survive for a longer period.
I'm keeping 2 pleco's with my reds for more than a year, 5 raphaels for about half a year. I have a group of cories and a clown pleco's in there as well, also for quite some time, but they are an easier meal than they ones I mentioned earlier, ime.