Both of my equines are on muzzles, and like you, I worried that they would overeat once I took the muzzles off, but this did not happen. How you use the muzzle will depend on the animal. My mini-donk is basically intolerant to fresh grass and so she wears hers from dawn to night fall and is still fat. We are thinking of putting her on a 25 hour muzzle.
My horse only wears hers from 7am to 2pm and this was enough to turn her from a total roly poly into a sleek beauty.
I know my vet usually starts overweight animals on an 8 hour wear time and then you just adjust according to what happens.
To get your pony used to the muzzle I found it was best to just put it on, pass food through the hole so that it know it can eat, show it it can drink and then let it go. The animal will figure the rest out on its own. When I started the donkey on the muzzle she was horrible and refused to eat through it. I was worried that she was not getting enough roughage moving through her system and worried about colic so I removed muzzle every three hours and let her graze for one hour. This actually prevented her from acclimating faster and she did not accept the muzzle until I just let her be.
Also, long grass and be very hard to eat through a muzzle, so the short might be better. Watch the pony and see how she does with that and then you will know where she needs to be. If you want to know which pasture is less likely to make her need the muzzle I think it depends on the grass length. If the grass is too short and too traumatized it will have higher sugars and you don't want this. In that case the longer pasture might be better, even though it has more grass in it.