On coyotes, it's a lot easier to get a combination that won't exit very often. On foxes, it's nearly impossible with the 223 not to have a bullet exit, so I try to use the tougher bullet method myself. I've tried the light weight polymer tipped bullets on foxes, and you'll be lucky if you don't blow'em nearly in half at any velocity.
I load 55g Sierra SPs at around 2600-2800 fps. I try very hard not to hit the shoulders(slightly far back in the vitals).
I've killed a lot of foxes with this combination, as well as bobcats(also easy to blow up), and although you can count on an exit, it's usually pretty small with very little pelt damage in most cases. If you do hit a shoulder on a fox though, it'll make a fairly big hole most times.