When dogs get into fights their adrenaline is ramped up. I wouldn't have taken him to another play date after the first fight.
I wouldn't even think of euth'ing him either!!!!
Take control and manage him. Is he in any structured classes? Do you crate him?
I wouldn't leave him alone with Annie unsupervised.
GSD's are not dogs that can just randomly go and play with dogs they don't know....epecially when new ones are added to the mix.
How old is Roscoe? Is he intact or neutered?