Molo is "aloof" to other dogs when out and about; and tbh, he is not close to his sister Bonnee, who we adopted when she was just over a year old last July

They do play/romp together for short periods most days, and he is
incredibly tolerant of her behaviour - despite her age, she is the Peter-Pan of cocker spaniels and still behaves like a puppy....he just accepts her mouthing him, and trying to steal his toys/chews/food, and if he really can't take anymore a quick snarl/snap and she realises that she has gone too far

They don't sleep together, and go off and do their own thing when out off-lead (well, Molo does his own thing, and Bonnee trots along after him like an annoying little sister

)
If you have pre-conceptions about the relationship between your existing dog and a new companion, then you may be disappointed; but if you don't expect too much, then a second dog does bring a new dimension to dog ownership
As for dog or bitch - I don't think it really matters, cos every dog is different

Is Millie spayed? That might have a bearing on your decision; otherwise, it may be wise to allow the breeder of your new puppy to meet Millie and help you decide the puppy that would compliment her
