What mix well with dogs boxer dogs? Seven months I have a boxer and wants to get her a companion, please help?

My puppy, Jess is very happy and playful and gets on great with all dogs they know, but I feel his departure on his own when I work, so I thought, if I have another dog that could accompany each other, but do not know what other races are mixed well with the boxers. Ideally, I prefer a smaller dog than a boxer as it occupies is enough space. He also gets excited around other dogs, because they are not used to other dogs, so I thought that if we got another dog later became more defendants to other dogs and not get so excited (and pull both) when he sees another dog on the street. Some people have said their dogs did not react well when you have a dog new, but most people we asked have said it is a good idea to get another dog as a dog is much happier with a partner. if anyone has any I would really appreciate advice. Thanks

All dogs get along with each other if they have been properly socialized.

Pit Bull, 2 Boxers and a Chihuahua