Hormones are a frequent cause of fights between dogs. With females in a normal pack the younger female would never get pregnant because that would be reserved for the dominant female which is the older dog.
In this sort of fighting normally a male would not get involved but if instigated might join in or if one dog was down he might join in the fight to help kill the downed dog so I'd not risk it.
The younger female will protect her litter from the mature female and this includes striving for female dominance in the pack and trying to kill any dog that comes near pups once they are born.
This is a great breed but they are bred to allow them to fight other dogs more easily so the social rules for dogs are more of a challenge to them.
I'd plan a private whelping area for the younger dog and to keep the other dogs away from the pups. As long as the older female is not in heat the older female and the younger male may be OK together.
I would expect you to have problems starting with every heat cycle and as the younger female matured around age 2-3 years old as that is when dogs typically challenge older dogs to move up in rank. The females may even fight over the male when coming into heat.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics
They cannot wear muzzles unless supervised carefully.
I'd set up the pregnant dog in her own space and keep her away from the other dogs.
She'll need that when the pups arrive anyway.
You can read more about pregnancy and whelping in dogs here http://lowchensaustralia.com/breeding/pregnancy.htm
If they use muzzles it would have to be the vinyl or wire basket type and you can see some here
http://www.pitbullgear.com/ProductDetails.asp?ProductCode=PitGear-7000
http://www.morrco.com/boxer-muzzle.html