What a wonderful thing you are doing! ^ I don't have experience fostering but have spoken to a few people who have and dealt with shelter dogs in general.
2) Some dogs take to riding in a car right away and Geno may be fine by himself. Some shelter dogs are routinely carted around in crates to adoption fairs so he may want to ride in a crate. The shelter should know. If neither of these, I would recommend riding with him in back, especially if he takes to you right away.
3) I've traveled with 2 dogs, no problem if they get along. They usually just find their own space in back. If they fight, all bets are off!
4) Shelter dogs have to adjust, unfortunately, to lots of different people. Being with a different person (Grandpa) may actually help him not attach to you so deeply that it would be difficult for him to adjust to his new forever family. If he gets socialized to new people, he may exhibit more outgoing behavior that will help him get adopted faster. It's not ideal but I would not worry about it.