Well, since we're being honest.
I have done it too, with disastrous results, so the idea of telling people it's o kay, is wrong, say it's risky, at the very minimum.
No amount of quarantining will fix a birds that is a "carrier" of a disease, you'll never know it had it. Chickens are bad about this, especially with blackhead disease and such as that... The chicken will never show any signs, you'll think you are in the clear, put it with gamebirds, and for ya know it, you have a pen full of dead gamebirds. So the "be careful" idea doesnt work there, as it would be a case of where you had no idea. Also, like my silver pheasant story above. They got along just fine, til that one day where the pheasant killed the merriam while I was at work....Cant bring him back, that was my fault for mixing. I ws careful, watched them very closely for several hours the first day, all was good, but it only take a minute for one to go nuts on ya.....
Again, OUR experiences are why we warn folks against doing it. Doesnt mean it's going to happen to everyone every time. But to say, they'll all be fine is misguided, as you or me neither one can tell what these animals are going to do for sure . But, it must be said and understood that these possibilities exist and are not that uncommon. Please understand, I am not getting on to anyone here, just want to be sure everyone understands WHY we tell folks this. A lot of the "new" people, dont know what you can and cant do, so we give our opinions in an effort to point them in the right directions for the most enjoyment and better health and happiness our of this new birds. But In the end, people are going to do what they want anyway, right?