Making meat balls in chicken marinade I have this marinade in with I usually make chicken legs. It consists mainly of maple syrup, soy sauce, and sometimes ketchup.  

Is it reasonable to put meat balls in that marinade? Is it OK to put them together with the chicken legs in the same pot?
You can use whatever marinade you like for meatballs, though I would personally find that way too sweet.

No you should not ever mix different raw meats together, unless they are being cooked and served together. That is called cross-contamination; contaminating one product with another.