That was one of my concerns as well and the main reason why we got a puppy. We have two male cats that are 1.5 yrs old and still very playfull. The dog is great with them, she just wants to play but unfortunately sometimes she doesn't realize her size and she scares the cats, but it has never in any way been malicious. The first 2-3 days we brought the dog home were pretty tense for the cats but they adjusted really well. We have also introduced the dog to horses, but this time it was the dog that was unsure what to think of this huge hairy animal in front of her.
Welcome to the group, and if you do decide to get a boxer I'm sure you won't be disapointed.