DH and I are looking to add another member to our family in the spring. We currently have a 19mos old neutered male boxer and a 10 yr. old female cat that get along pretty well (she draws the line and he respects it

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While I would love to have another boxer to complete the "bookends," I'm not sure I can take another big-ish dog in my house- although it's really tempting!!
What are some other smaller breeds that get along well and have energy level of a boxer? I've been leaning towards a Boston Terrier because Jake has some BT buddies that really give him a run for his money!