Greys and greens? I do it all the time. Just take a look at my avatar. I've kept greys and greens together successfully in 1970's, 1980s, and this decade so it transcends the realm theory and conjecture and has withstood the test of time.

I will also add that I have never seen any examples of untoward inter-species aggressiveness between the two. Some individual frogs are of course more aggressive than others and I've had aggressive greens and aggressive greys when it's feeding time.