Your 5 month old Pitt is very much still a puppy...she still has another couple of years of maturing to do. Often when Pitts (and Rotts) reach sexual maturity ...(does not matter if they are spayed)...they may decide they want to kill one another.
You have no idea what things you are still going to face with your Pitt puppy...she has still not finished teething.
You still have to get through the teenage stages...and Rottweilers are even more hard headed and pushy then Pitts.
Please re-think your plans. Why do you think you need to add another puppy to your home now? when you have 2 puppies? Why not wait till your puppies are all grown up and trained. What will happen if your Pitt does turn dog aggressive?
Most Pitt people will warn you about this...it is common in the breed.
You can train and socialize all you want...but as your female matures...she may decide that she hates another dog...what will you do??
Gina