Either sex would work. I currently have 1 male and three female GSP's.
Our third GSP female was out of the box a HIGH energy pup. We just threw her right into the mix with the older dogs.
The older dogs literally observed her for an entire day or two, and then they were like "Okay- welcome to the club". This is an exceptionally tolerant breed though.
Our 4th GSP actually came from a litter that our #3 had, and the guy that bought her turned her back to us. (She was the best pup of that litter too- originally I wanted to keep her so worked out in the end).
We got her back at 1year of age and we had to keep her and her 'mom' separated because they would fight like hell. They calmed down after we got them both spayed.
So we just watch the interactions and go from there. I would also recommend you speak to your vet if you get a new pup on the spay and neuter recommendations. Most vets and NAVHDA folks recommend males get neutered after a year so they fill into their bodies. Certain breeds can get 'leggy' if neutered too soon.