Depends on the horses. Whenever introducing a new horse to my 'herd', I
always have them in an adjoining pasture or corral so they can get
acquainted over the fence. Sometimes it takes just a few days, sometimes
a couple of weeks, and sometimes never. Play it by ear. If and when you
do introduce them in the same field together, be referee for a couple of
hours, just in case. I had a new mare run through a fence by my gelding,
severed an artery and died before the vet could make it there.
One thing I've learned is that horses seem to prefer getting beaten up than
being my themselves, and usually they work it out OK. I think you'll find
in the long run that the mare will be the dominant one of the group.
Good luck. Joyce