
There seems to be a pre-concieved idea that certain mixes of sex do or dont go well together. I would say that it all depends on the temperaments of the dogs in question. I would consider the following:
1. How are your dogs with other bitches normally?
2. Would it be possible to let them meet and have a trial visit in your house to see how they get on?
3. Your 5 month old is still very much a baby, would you still be able to give her the training & time needed?
My sister has 4 ex racer Greyhound bitches, she started with one and then another and so on, they are wonderful together and she has never had a problem. She was told by her vet that you can't keep multiple bitches together as they would fight. Another friend of mine has 7 bitches and one male and has no problems either.
Hope it works out for you whatever you decide.