I would not put a zebra in with them, but there may be other options.
Personaly I'm rather fond of clown pleco's. But they do need driftwood (fiber) to much on to maintain good health, and depending on your water, size tank and ammount of wood this might end up more negitaive for the tangs. I say could, as I really don't buy into the hype that drift wood will have a massive effect on water chemistry. If your keeping tangs at the desired Ph/GH/KH then I would say that it's not really going to do anything at all. But in theory it kinda works...
Do you want a pleco, or are you just looking for something else to go in the tank with them? Check out some of the Syno. catfish. Several species are fairly available that come from Tanganyika. I have some S. multifaciatus (A.K.A. KuKu Catfish) and they are by far some of my favorite fish to come out of the Lake, even more than most of the cichlids. S. petricola is another common species that many people adore.