I thought you meant alpha as in queen, top of the pecking order, then you said beta - so then I'm not sure what you mean.
There is really almost no way you will know if the betta you buy will be more or less docile. Even initially they don't show their true aggression, and sometimes not even until they're settled in their new home.
I've found that the largest of my females tends to be the alpha, but that is not always the case. Finding a girl that is at least not bigger then the alpha you have might be better.
I would not add one female. You might do better by adding more at once. If you do only add one - move everything around; otherwise I really would suggest adding 2 or 3 at a time.