[Marilize]

In general, the best combination would be to add a girl. Both should be spayed/neutered, of course.

There are exceptions to the rule, but in general, they tend to be more at ease with each other if they are different genders. Also, if you have a dominant little guy at the moment, make sure that the new puppy is submissive. This way, you can avoid tension and fights over 'who is boss'. Once again, there are exceptions, but this worked really well for me.

I got a girl after two years with a boy. He is dominant and she is submissive and they are best friends. She still irritates him sometimes, but he loves to play with her. And there is no funny business, except for him teaching her some manners.

Good luck with your choice. All dogs love having a buddy!