A gut supplement will be fine to feed alongside what she is already having, I have used Easigut from natural horse supplies although it wasn't for ulcers his just sensitive but it is recommended for ulcer prone horses.
She is looking good definitely a bit slimmer I am sure she will grow a bit more, Arabi was about 14 .1 at 3 and fully grown his just 15h, I think Arabs tend to ride bigger anyway so they never feel small or pony like.
I wouldn't leave stallions with a mare carrying and definitely not with a new born foal, I have looked after stallions had one myself most are very good but you just never know, we ran colts together so they could play and grow but if one got too aggressive they were separated I can't be...
Well that's not a happy herd the stallion should not be subjected to bullying everyday that is just cruel, and she is obviously not feeling safe and happy or she wouldn't be so aggressive towards him, not my idea of how to keep horses poor bloody foal being born into that situation that's quite sad.
We tied the muzzle to a field safe headcollar it was the only thing that worked, one of my Arabs got very good at removing it from the pony and he quite happily stood there while he pulled at it, it's like he just knew that he would be free of it.
I pay £10 for each visit twice a day, hang nets 1 bucket feed either end if the day, top water up make haynets and feeds and maybe change rugs if needed, it doesn't take any more than half hour and she only lives about 10 minutes away.