Mostly, I tend to keep like with like when i make stock. 
Beef with beef, lamb with lamb and so on...but in the end its just bird with bird lol But I do add vegies to my stock, and a little vinegar/lemon juice to get all the minerals out of the bones. When it comes time to strain the stock, I just squish the bejeebers out of the veggies so every bit of juice that is in them comes out into the stock. My hubby is not a huge veggie fan so I have to try and get it in where I can with him. He really doesnt noitice when I puree the carrots into the spaghetti sauce, or add pureed veggies to the stews <snicker>
Turkey and chicken, chicken and quail, chicken and chicken....if its a bird it goes in the pot. I dont really see that much of a difference tbh when I make the stock. The beef to me is very diff than the turkey etc, and so on, but the bird stock really is not a worry to me if its mixed. The idea of beef and turkey tho... makes me go ewww lol