tiger barbs and albino corys is normaly bad, I would not chance harming the expensive Discus either
One small fish I really like is the Espei rasbora. They are much brighter colored than the Harliquins and stay a little smaller. The Hengeli rasbora looks a lot like the Espei and Harliquin, but I think the Espei is the nicer looking fish with much more neon orange coloration.
Cardinal tetras are one of my favorites, but I wish they were a little less skittish
You can always mix some livebearing fish like a few swordtails with your fish as well. The live food they provide to other fish is beneficial
Another neat fish is the dwarf neon rainbow