No, they should be fine. For years I had Green Giant(big male greenie), Emerald(female greenie and GG's mate), and Lizzie(sassy little female brownie)together in the same cage and they all did fine. Just don't put a baby greenie with an adult brownie. Brownie's have been known to eat(yes, I said eat)baby greenies. I wouldn't worry about interbreeding between species, because it's rare. But if the species do interbreed, I don't think there would be any danger... why the heck would there be danger I'd love to see that!!!