The kribs will probably be ok,i know people who keep appistogramma dwarf cichlids with discus without problems and they are quite alike.The RTBS would be a no if you ask me,discus are easily upset and a aggressive shark may be too much for them,discus are expensive fish as im sure you know and if stressed quite often give up eating and die.
Other good tankmates for discus are corydoras catfish and L number plecos as well as smallish tetras like cardinals which appreciate the same higher temperatures,very small tetras like neons may be eaten.
The main thing to remember is that discus need very calm non active tankmates and those that allow the discus to have the lions share of the food,discus need to get a lot of high quallity food to keep them at there best.