[SWHouston]

My experience has been to allow the resident to mature, and confidently be integrated into your Fam. I suggest that you wait until it is 2 yo, before you introduce another. I've had very good experiences bringing in a Puppy to an established Adult pack environment. The puppies have been accepted as though they were their own.

Then...
Same Breed,
Opposite Sex.

Be sure to be very attentive to your resident, any introduction like this can have very troubling effects. Greet, pet and feed it first, and intentionally allow privileges above the newcomer. Don't worry about how the newcomer feels about this, it expects to be 2nd given the circumstances.