It all depends on the individual dog and how strong his prey drive is. There are Boxers who absolutely cannot be left alone with any smaller animal (other than another dog). Our Boxers have been raised with our cat and to them he's just one odd looking Boxer. Even the cat thinks he's a dog. However, having said that, if Chino is outside and Penny is also outside, as long as he does not run from her everything is kocher. But... if he runs then the chase is on and there's been times that we've feared the worst.
Friends of ours, who also raise and show Boxers, also have 2 cats. The cats cannot be left to roam the house when the dogs are inside. It would not be a good thing. Funny thing is, is that our cat is a brother to both of these other 2 and yet their temperaments are totally different. Then again, Brooke, our eldest Boxer loves ALL baby animals/birds and when we brought Chino home, she took it upon herself to be his mommy (see attached photo).
The best thing would be to crate the dog and allow the cat to roam free and see how it goes.