I think that you can mix the groups, but be sure to have enough of each to make for good group behavior.
Cardinals and serpae tetras work well, the serpaes can't catch the tetras but they keep trying to intimidate them. The cardinals school some, when moving from one side to the other, but mill about in the plants individually.
Mix another group of bottom dwellers, then a group of top dwellers if you have room after the cleanup crew.
Member AGA, HAS, NASH
110G, 10 adult discus, barebottom with plants on wood
58G, 3 adult discus, barebottom with plants on wood
59G thickly planted tanks, rummey nose tetras, kuhlies
300G goldfish pond outside