You can't really guarantee that larger cichlids won't eat smaller tetras. Even peaceful fish like Uaru and Mesonauta will hunt small fish when they're hungry. I've had cichlids that left small fish alone, and others that eat anything tiny. It's got a lot to do with the individual fish. I've also had discus wipe out cardinals!
One thing to keep in mind is that your discus should be in a group of six or more to feel comfortable, and they should be adult size before you introduce them to other large fish. I have kept discus with both Uaru and Mesonauta, and several people keep discus with more peaceful Geophagus like the orangeheads, but you need a large tank and a quiet spot for the discus to get away from commotion if they're feeling stressed or spooked.
My Mesonauta nipped at the discus a bit and made them skittish, so I ended up getting rid of them. U. amphiacanthoides did much better.
You could probably keep them with Heros as well, provided that your particular individuals weren't too aggressive. I've kept discus and angels with severums and it works out well as long as you have a passive severum.
It's really going to depend on the size of your tank. 150 gallons and 500 gallons are worlds apart. I wouldn't mix discus with Uaru in a 150g, for instance, because you're dealing with two species of large-growing, shoaling fish and you'd need more space. But a 500 gallon would be great. So the final size of the tank will narrow it down further.
