Discus require 45ltr/10gal per fish to grow to adult size and be healthy.
The minimum size tank for discus is 100ltr/22gal, thats to house an adult pair no other fish.
I dont recomend barbs and guarami as there not to comftable at higher temps, the usuall suspects are cardanals, rummynose, penguin, glow light tetras and a few more besides certain cats, rams and apistogrammas also do well, what ever you add muct be peacefull and able to stand higher temps.