I have a male Cavalier King Charles Spaniel (1 1/2 years) and a Yorkie mix (9 months).
I really agree with 2Parises advice. You must be honest about your current pup's personality. My Cavy, Louis LOVED other dogs before we got Ella and is the most laid back, submissive dog I have ever met. We got Ella Bean from a shelter and while I didn't know much about her personality, I had a background of babysitting other pups with Louis and he loved his 'playdates.'
They do kind of work it out on their own and Ella is definitely more dominant/ agressive, but there are still fights, mostly over chew sticks, the one thing Louis doesn't like to share.
Go slow, if your first baby sleeps in bed with you, don't invite the newbie in bed right away, I think dominant/ submissive personalities are more important then a male/ female, but if you get a male, I'd have him neutered ASAP. the more testosterone, the more fighting. Boys will be boys!
