You have to be very selective on the species of corydoras, as the high temperature required by discus cannot be tolerated by most species. There are a few that will, such as Corydoras sterbai, C. leucomelas, C. gossei, C. seussi, C. adolfoi, C. duplicareus come to mind. The majority of species prefer cooler water, by which I mean mid 70's, up to say 77/78F, and will struggle with the above 80F temperatures that discus require.
Cardinal tetra will be fine with discus temps, though I would reduce this number to around 12. Corys at 12 is OK. Discus I would say 5, but if one of our members (discuspaul) says differently, I won't argue.
Byron.