Most tetras are okay if they're not fin-nippers. It's also best to keep them in a large enough school that they nip at each other and not your discus. Check their temperature requirements -- some tetras come from more temperate southern areas and don't like the warm water discus require.
All I can tell you about any sucker fish, including ancistrus, otos, and other plecos, is that they sometimes develop a taste for discus slime coat and will rasp the slime coat off your fish. This is an irritant than can make your discus jumpy or nervous, and could also leave them open to infection if the water isn't kept clean and the plecos rasp off some scales. Ancistrus and otos are the safest choices when it comes to sucker catfish, but there have still been plenty of stories of them eating slime coat. I've seen albino and brown ancistrus go after my discus multiple times and I no longer keep them mixed.
Amano shrimp may be a very pricey snack for large fish. Discus have no problem eating shrimp or small tetras.