Stopping our gelding from mounting our yearling filly

Question
Hi and thanks so much for this website!
Here's our dilemma.  We (my dughter and myself, mom)recently brought in a one year old Friesian filly to live with our 18 year old quarterhorse mare and 5 year old quarterhorse gelding. They welcomed her nicely as we expected. Within a few days, the filly woke up something in our gelding and he was constantly trying to mount her.  He brought her into heat at a year old.   She was also very encouraging and wouldn't leave him alone. She's a large filly but he's about 600 pounds  more than her and we were afraid of injury to her.   

We've had our gelding since he was a weanling (6 months old), trained him and he's been a gentle nice horse with no problems in getting along with our 18 year old mare. He's been with amother older mare for a few days, within the last year and has shown no sign of wanting to be "romantic".  

I'll describe his gelding circumstances.  He was a cryptorchid when we went to geld him at a year and a half old and he needed surgery to remove both testicles. He hadn't been around any mares  except during his first 8 months and was too young to think about mounting. He "roomed" with a couple other young boys his age until his gelding. We've recently had him blood tested and he is definitely fully gelded.  His behavior was that of a  stallion with our new filly.  

We immediately had to separate him from the "girls" and has been in a small, mostly dirt pasture with no one for company and no other horses are visible to him from where he is.  We hate this and want him back with his old mare friend and the new filly.    Due to circumstances, this is the only option at this time.  We did put him in another pasture at first, where there are 4 older mares and he went after two of them, trying to mount them.  One of them became encouraging and this was a 7 year old mare who stayed with him and our older mare, about a year ago for about a week and nothing happened.

We've recently bought a ViceBreaker to stop his mounting and want to know at what point to use the electric stimulation.  We don't know if  we should use it at the first point of him and the filly sniffing noses which is usually their first contact and she's gumming and he starts his low stallion-like talking or wait until he puts his neck over her back just before he tries to mount.  We don't want him to be afraid of her but to just know he can't mount her.  Our vet said to use it when he starts to mount and there's a fine line there.  We don't want to use it when he's already covering her because of possible injury to her.

I hope you know something about this sort of technique.  We can't be there all the time but are with them all weekend.  It's supposed to take only a couple to a few stimulations to break the behavior.  We have to be watching and the timing is what's important.  Can you help us?!

Answer
      Hi Gale,
   I wasn't sure whether to attempt to answer this question at all as it is not really within my area of expertise, but I was curious about one particular aspect of the situation. I always believed that a male (stallion or gelding) that approached a female, who was not in season, with "romantic" ideas, would be very firmly put in his place. Are you saying that even when not "in season" your filly will allow him to mount, or even attempt to mount her?
   As regards the ViceBreaker, I think I would be inclined to use the first stallion noise he makes as the trigger. I don't know if there is a time lag between them meeting up, and him starting his stallion act, but if there is, it would be ideal, as you could allow them to be friendly, and only introduce the deterrent when he starts to show a more amorous intent.
   By the way if you "Google" ViceBreakers, you may find articles by others who have used them, for various reasons, and you might find some of what they say useful.
   Another thought that occurred to me was, whether or not it would be possible to give your gelding his own "room" within the same paddock as the two females, through the use of electric fencing.
   I don't know if anything I have said helps, but I would like to hear your thoughts, especially as it might mean I would have to revise some of my ideas.
   Meanwhile I'll wish you luck.
       Slan,
          Brendan