Cardinal, rummynose, serpae, lemon, and many others will be fine. There are two issues here 1-the exact temp you keep your discus at and 2-most tetras are from Amazonia. 1-Discus can thrive at anything from 82F to about 89F, the exact temp you keep yours at will greatly effect which schooling small fish will thrive in your tank with them. 2-Many people think of discus as if they are from some remote lake that is so hot and acidic that no other fish naturally live with them. The truth is they are from Amazonia like HUNDREDS of other fish that are in the hobby. Most of the fish from Amazonia are compatible with discus. The exact ones will vary based on the temp you keep your discus at.
When in doubt, rely on fishbase.org for accurate information about natural parameters, max size, etc. The site is NOT hobbyist based, it is used and maintained by scientists so the information is a lot more accurate than what you can find on hobbyist sites.