Broody hens will hatch bantam and full-sized fowl chicken, pheasant, turkey, and about any other poultry eggs and raise them, even if the eggs are mixed as long as they hatch at the same time. I had a hen hide a nest and hatch 18 chicks. She raised them all. I've given a hen incubator chicks in addition to what she hatched to get her up to 15 total. Again she raised them all. I would not be the lease bit concerned about the number or size of the chicks you are trying to hatch.
It sounds like you could have what we call a staggered hatch. That's where the chicks don't hatch close enough together so the hen has to decide if she takes the ones that have hatched off of the nest to find food and water and abandon the unhatched eggs. The majority of broodies will decide on the ones that have hatched, as they should. Since the chicks absorb the yolk before they hatch they can survive for over 72 hours without eating or drinking. Some broodies wait that long to take them off of the nest but many don't. Staggered hatches can be very stressful, I try hard to avoid them. If you have problems with a staggered hatch, that's on you and not the broody hen. Don't do it again.
In addition to the potential problems of a staggered hatch, some hens are like your mares, their instincts may not be quite right. I had a broody hen kill two of her chicks that she hatched but did fine with the other six. I have no idea why she killed those two. That's the only real problem I've seen myself but some people on this forum say they have seen things like a hen that would never take her chicks off of the nest, might peck at eggs that are hatching, kill all of the chicks, or do other things that are not right. I let my hens hatch with the flock and have never had another hen kill any of the broody's chicks. Some people I trust say they have seen that. These types of problems may be really rare but with living animals you cannot be sure what will happen.
The potential staggered hatch is the only red flag I see with yours. Sounds like it is going great otherwise. Good luck!