I was up against this just a couple months ago. I've found they do better in pairs. Even if they are just "getting along" they still have another one around. Three times now we've introduced one of our Basenjis to their new playmate:
Our first, Brie (female), was just over 2 when we brought in 12 week old male, Ramsey. Brie never fully accepted Ramsey. I think she would have preferred to be an only dog at times, but they did occasionally play. Still, she made it know this was HER house he was in. They coexisted for the most part, but never fought. 12 years later after we lost Brie, we got 4 mo old Bonnie. Bonnie loved Ramsey, but now the roles reversed – Ramsey got tired of Bonnie's puppy play and nipping -- WE got tired of his reaction! We wanted to break this cycle. So 6 months later we visited a breeder who had 2 puppies and 2 9mo olds. Bonnie DID NOT like the puppies, was OK with one of the older males and wanted to play with the other older male. So in came Smitty. Bonnie may have warmed up to the pups but she really bonded well with the one older male. Bonnie and Smitty are the same age, opposite sex and are now best friends.
Given my experiences, I'd say sooner is better if you know for sure you're going to get another. Definitely opposite sex. If possible, take Alani with you to see how she reacts to another dog, I'm not sure how Bonnie and the other male would have bonded. Give yourself enough time to let the dogs get use to each other. We were at the breeder's for a couple hours.