I adopted my female boxer at 7 months old. 6 weeks later I knew that I would need a second dog in order to stay sane! :crazyeye:. She just had way too much energy, and was always looking for us to entertain her. So her and I went to the shelter where we got her and interviewed 3 dogs and came home with the one that seemed like the best fit.
There was a cute male lab mix but they went crazy together playing and wrestling and I didn't think we would be able to handle all that energy in the house. They were so intense together that it was hard for me to control them, right off the bat.
By chance they had a small little guy - he seems like a maltese or havanese poodle mix. He was 3 months old and they were great together. Interested, playing and hanging out, but manageable. It has been a fantastic fit. They really complement each other. They play all the time, they hang out, and they genuinely like each other and enjoy each other's company. Plus my boxer helped me potty train him! Since the second dog is little (12 lbs at 1 year old), my boxer has learned to be more gentle, and I can walk them together easily. Also, my little guy is very sturdy and healthy, so I don't have the issues I've had with my boxer girl.
I would say think about what kind of companion would best complement your dog and your household and then seek that out, regardless of breed. I love my little boxer girl, but honestly, she is very high maintenance (both in terms of time, energy, and finances). She has been the most expensive dog I've ever had, due to her health needs, and resulting vet visits, diet changes, seizures and the very expensive testing for that, etc. We've also had to make some expensive changes to accommodate her energy needs and play styles.
I know lots of people here have two boxers and that works out great for them, but you want to think of what would work best for your, your family, your schedule, and your dog's personality. I couldn't do two boxers, myself. But I could see maybe a mellow rottie or mutt to play with but that would help my boxer chill out.
I also find that since my dogs have a companion at home, they don't go crazy trying to get to other dogs when we are out. They'll just walk by other dogs and be on their way, even off-leash. I find that I don't need to tire them out as often (for my boxer especially) because they're playing most of the day together. As I type this, they are wrestling and chasing each other around with a squeaky toy.