I adopted a rescue female for my boy as a playmate. They were both close to the same age and did great. They were close to a year and a half when I did this.
I have also (she passed) added a puppy (boy) to my male's life and he is now 6. They are great together as well. The older one takes care of the younger one. Their play levels are different, but the relationship is great as well.
It is truely and exciting experience to get to know the opposite sex of this breed. They are a little different in their behaviors. The girls are a little more growly to talk with other dogs. Boys together tend to box more and do well together with all other dogs.
Keep us posted on what you decide!! Your basenji will be so happy with a buddy female or male…..