I agree with Binka, and yes, females are generally more of a fighter than males, for some reason it is especially so when both are spayed.

Most shelters & the such will allow & reccomend the dogs meet first, try to do this on neutral ground (park, someone else's house etc..) although even if they get along there does not guarantee that they will get along once at your house. Although my rottie is mostly the opposite, she is awful friendly to any other animal in house, but once outside she is weary. Maybe make an agreement that allows you to return the dog after so many weeks or months should something occur & they will not get along.

Because of your dogs age & the fact she has always been an only dog it would be best that you not interrupt her schedule at all. Make sure she gets all the attention, food, walks, etc... first. It may also be easier on her if you get a dog that has the same energy level & approx. age range.

The fact that your dog is alpha & the other is smaller & more submissive is about a 50/50 prediction. These facts may help them get along better. Or it may cause trouble. Your alpha may sense that & become more aggressive cause she knows the other dog is submissive, she may try to push bottons. But the fact that the other dog is smaller she may be apt to become more injured.

I personally think it would be best for you to think about adding a male dog about the same age &/or especially energy level. Of course if you have arrangements for the female dog if they do not get along I would give it a shot at least.

Maybe speek with a reputable trainer, your vet, a behavorist etc.. on some pointers & maybe classes you can enroll in. Good luck!