Small and large dogs can get along totally fine, but the smaller dogs may also very easily be injured in innocent play with the larger dogs.
It definitely depends more on the individuals themselves than the breeds, but that said, Pits and other Bully Breeds tend to be VERY rough players and also tend to have a fair amount of prey drive. Also, dog aggression or at least selectivity IS a common trait in Pits- breeding them for dog fighting for so many years has made its mark. They can be really great dogs, but I really hate this "it's all how they're raised/it's the people that make them dangerous/they're the perfect dog for everyone". There are a lot of breed traits that skew them towards being a lot of dog to handle- they tend to have A LOT of energy, especially when young, and also tend to be pretty smart, and that can be channeled into frustrated aggression or destructiveness; they tend to be excited by small running things and have a strong prey drive, which means that they like to chase and can have trouble with other animals (ie, cats, rabbits, other small pets) and small dogs, and as I said they have a very rough play style which means they can sometimes hurt other dogs by accident and will sometimes get into fights because most of the young ones I've met aren't great at respecting other dogs boundaries. They are definitely not the easiest dog to own and IMO they are a breed where good training, management, and boundaries from a young age are important or else you end up with issues as they get older. You are probably going to have to manage the pit puppy carefully with the other dogs or she might hurt them accidentally.
I too would be more concerned with the number of young puppies in the house more than the fact that there are large and small dogs together. As said above, littermate syndrome is something to look into and the dogs don't need to be related or the same breed or size for it to be an issue, just the same age.
You already have 2 very young small breed puppies and now have a young large breed puppy. All three of these guys need to be individually trained and exercised and IMO should be spending as much time alone as they are together, both in public and at home. They need to know how to spend time alone, and that not being with the other puppies isn't the end of the world. Given they have 3 different owners, this will probably be easier than if they all belonged to just 1 person, but is still isn't going to be easy.
I would NOT recommend bringing another dog into this house, especially not a puppy, especially not a giant breed puppy, and ESPECIALLY not a Great Pyr, which belong to a group of dogs called Livestock Guardian Breeds and have a lot of harder-to-live with traits such as protectiveness, aloofness with strangers that can turn into aggression if not well socialized, are often aggressive to other dogs, and tend to be difficult to train because they are so independent. They are one of the easier guardian breeds to own, with most lines being bred for pets and not work, but still have the traits that make them a difficult group.