I think many will agree that there are 3 basic types of fish when it comes to movement. You have the danios & barbs that are very fast and high energy. You have the moderate-semi schooling fish like tetras and some cichlids, and you have the slower, graceful, stately moving fish like angels, gouramis and discus. This is a generalization but covers the basics.
It really depends on what type of "atmosphere" you want to create for your tank. The danio group is high speed, all over the tank like a flash mob moving through. Mixing them with slow movers like can really stress the latter and keep them from their normal behavior.
I just kept nine zebra danios for my daughter while she redid her tank for about 5 weeks. My tetras would not do their normal schooling because of the danio "flash mob" moving through all the time. Now that the danios are gone my tetras are back to schooling nicely at the front of the tank again. Hope this helps. OS.