Is your 10 month old boxer neutered - if not that could be your problem, but if he is and he still has problems with Male dogs then you would be safer to get a female.
Introduce the dogs on neutral territory, it might even be a good idea to let them meet a couple of times before taking the new dog home.
With regards to leaving them alone together unsupervised this is not advisable even if they appear to be getting along together. They should be crated or left in seperate rooms as you don't know when a fight could break out and could get ugly if you are not there to seperate them, I would wait several months before taking that step even if they appear to get along well - you just never know.
If you are getting the Rottie through a rescue they will help you with the introductions and help you find a dog that will get along with your boy.
Good luck