Pleco's come in all sizes and colors, but your best bet would be a Sailfin or Common pleco - they're cheap, readily available and tough.
More fancy ones can be very costly, and will most likely end up as an easy and pricey meal for your piranha's.
I don't know what size your reds are, but a pleco over 6" should be able to hold its own, if he has a feew hiding places. Very large ones (10" and more) will have even more chance to survive, but are messy, and will crap as much as they clean up.
And always keep in mind that
anything kept together with piranha's is a potential meal - don't get attached to any piranha tank mate!
I've kept a number of different pleco's with my reds, and only my Sailfin survives (he's with them from the start and grew from 3 to 7,5", so that might explain why he's still alive...)
Here's some pics pics of some of the pleco's I kept with my redbellies:

Goldnugget Pleco (most expensive feeder ever - survived for about 4 months)

Snowball Pleco (got it for free - lived for about 4 months also)

Common Pleco - lived with my reds for about 22 months and died two weeks ago, for no appearant reason

Sailfin Plec - still doing great, since day one!
To wrap things up, here are two links to some the finest website dealing with catfish:
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www.planetcatfish.com-
www.scotcat.com