A Boxer might be a little too much dog for your Rat Terrier- I would suggest an adult rescue if possible for you, so the dogs can meet first. A Boxer puppy or a dog under 3 may be a little too exuberant for such a small dog, but since I am a fairly new Boxer owner myself, I would suggest joining a Boxer forum and ask what they think, and what kind of dogs they have with their Boxers.
Our Boxer plays well with puppies, but she is also pretty laid back for a Boxer-LOL! I have noticed that several Boxer owners also have Boston Terriers, but I don't know their situations
. Mattie is our first Boxer, after having a Cardigan Corgi for 14 yrs- it was a toss up which we were going to get. We finally decided to get whichever came up first (from rescue, etc. since we wanted an adult) and Mattie showed up first. She is a wonderful dog!
If you like the idea of "Big dog, little body" a male Cardigan might be a good dog for you. They are just that, about 35 lbs (more or less), personalities alot like the Boxer( it was really funny reading the personality descriptions-they look so different, but are so much alike in their brains!), but more of a people pleaser (therefore, easier to train!
), and much easier to pick up when needed
. Good luck with your deciding!
(If you want my Boxer forum address, I can pm it to you