There are no limitations on where you can play female players in Mixed in USSSA according to the national rulebook. If you wanted to play an all male infield and all female outfield it would be legal. It is a common local rule to require two female players in the infield and outfield, just like it's a common local rule to have a restraining line. At the national level, you can play 8 infielders if you want, anything you could do in the field in men's or women's you can do in Mixed.
It's very hard to find a rule that allows you to do something, most rules restrict you from doing something. I would ask where in the rulebook it says that you can't do that. Depending on what state you're in, you may need to take it up the chain to the state UIC, Regional UIC, National UIC or National Director of Officials. You can contact me if you are having issues. I leave for FL next week and will be seeing and speaking with all of the Regional UICs as well as the National UIC and Director of Officials.