I think everyone here has covered it pretty well so I don't have much to add. I agree that the PBT and the blue hippo may not be so complacent with each other in the tank but I would imagine if you introduced them at the same time and they were roughly the same size, it should be possible. The hard thing, as Sascha hinted to, is that both are highly prone to ich and other diseases (add to that the Copperband), and you have a potential recipe for disaster because if one gets sick, they'll all more than likely catch it. I would also recommend introducing them last... just ask Sascha about is PBT... and how it'll attack anything new in the tank.