Crate and rotate. NEVER leave them together unsupervised or just out of reach to prevent anything serious. There's a saying; "Males fight for breeding rights and females fight for breathing rights". Its not a joke. Sounds like you have too many females under one roof. Its usually recommended to have alternate sexes when you have dogs as there's less chance of fights. Female GSDs can be pretty dominant towards other dogs. Males tend to be a bit more laid back.
I currently live with my dominant minded female (now my father in laws dog) and my less than dominant minded female. Zena doesnt generally tolerate other females for extended periods of time and Shasta just wants to be friends with everyone. I keep the two seperated except during potty breaks when they are in the yard.
Definitely keep these two seperated from one another. Fights ALWAYS gets worse and considering a GSD can very easily snap the neck of a smaller dog, unless you want to see your Boston broken like that, these two need to stay away from each other.
The more people I meet and talk to,the more I love my dogs and their intelligence.
Zena GSD 6/1/03
Shasta GSD 5/5/10
"Dax" Thor z vom Weberhaus GSD 3/18/2013
Riley GSD/BC 1/10/05-2/1/2013