I hear what you are saying, and I understand that they shouldn't be mixed. All I can tell you is, that here in France you can walk the countryside and see gardens full of birds - turkeys, chooks, gunea fowl, ducks and geese, all living together. Everyone here buys in their 'stock' for the year at around June time and slaughters before Christmas. They do it year in and year out with ill effects. We even have the mix and my conclusion is that on a small scale I don't feel I have a problem. The French live long lives, so I feel confident!
Kate