The makings of a peaceful clowder

So I have mostly a peaceful stray colony...aside from a mother and two kittens I rescued from underneath my neighbors house a few months ago.  Two Toms and the mother have been spayed.  The kittens are scheduled for next week.  My problem is the mother of the kittens.  She seems to want to kill a spayed female I've been caring for for over a year.  I have an outdoor enclosure but the cats are not enclosed...they have free reign of the backyard and the woods beyond.

So far, I don't have any neighbor issues or reports that these cats are offending anyone.  I would like to give the free reign permanently if I can get the two females to see eye to eye.  I have provided separate feeding areas and they have a huge sand box in the yard.  Plenty of different wintertime lodgings....is there anything I'm overlooking?  For now, I am controlling their meetings by only letting the mother and family outside at specific times of day.  I'm ready to turn them out full time.

Can the mom spend her time inside the enlcosure?  We have a cat who absolutely hates other cats, so she has her own room.  Perhaps this would work for these two ladies, as they both appear to want to be the boss.

Or - can the kittnens join the clowder and and you keep mom as in inside only cat?

Otherwise, I would definitely take the time to make slow introductions between these two.

Good luck!