Standard Focus wheels are ET52 (so the hub face is 52mm away from the centreline of the wheel), I can't remember what offset the Mondeo wheels are but you should not, generally speaking, go lower than ET42 on a Focus - wheel width and tyre sizes will affect that though so you could in theory go a lower offset with a skinnier wheel and tyre but probably not in your case here.

I'd suggest it's the tyre causing you issues at the moment.