You might want to try Farrowella Cats (sp?). Mine has lasted longer than any plecos. From what I can gather, I think it has to do with movement. Plecos and other cats I have tried seem to make "excited" movements with their tails, especially when feeding or excited by alot of movement. This, in turn, excites the pygos and stimulates them to nip. The Farrowella seems to be very cautious and usually stays very still when the Ps are near him, even if they are feeding next to him. He once got a small nip on his tail, but this was due to the current of the powerhead pushing his tail around.
The shape may also deter them from biting since the Farrowella doesn't really have a "typical" fish shape. It seems to clean up algae pretty well, but I wish it would bottom feed a little more. I am thinking of picking up a few more and adding a couple snails. Just my .02 from observations in my tank.