I have mixed and matched and haven't experienced an issue with my Cherokee (Toyota transmission).
It specifies DexIII/Mercon.
When I bought it, I did a drain and fill with Mercon V fluid. Have done a few drin/fills with leftover Mercon V then, when I ran out of that I used Supertech DexIII fluid. Last trans fluid change was with Mobil ATF D/M.