Good tetra tankmates include Cardinals, Rummy-Noses, Lemons, Glowlites, Head & Tail Lights, mature Embers, and some other peaceful, slower-movers as well. Serpaes may be a problem (occasional nippers). Some keep Green or Black Tetras with discus.
Almost all types of Corydoras are just fine, excellent the more delicate Pandas, which may have trouble with the higher temp.
Several types of Rasboras are a-ok, as are Hatchet fish (but Hatchets are jumpers, so you need to keep the tank covered for sure).
German Blue Rams are great tankmates, as are BN Plecos (but not most other Plecos).
Some keep Otos and SAE's, but keep in mind that at times, some of these can take a liking to discus' slime coats as part of their diet.
Some keep Angels with discus, but that can be potentially chancy. Others keep Guppies, Platies and other livebearers, but of course all the fry usually get eaten by the discus.
There are other species as well, but names escape me at the moment. As I said before, just takes a little homework if you're unsure.