1. I'm not sure on neuter/spay, all my dogs have been by the first year, I know differently now, so Ranger gets to keep his, at least for the time being. My lab went into heat early and what a mess, for me I would at least get the female "fixed".
2. Some dogs don't get along, but more than likely, if you start with a pup, they'll be fine, at least that's been my experience.
3. All three of my gsds have different personalities. Tuke is laid back and enjoys relaxing with the entire family. She'll lay on the bed with me or the kids and is just fine being used as a pillow. Zoey, Tukes litter mate, is serious, lots of drive and energy, she will go go go if you let her. She enjoys attention to, but with a time limit. Ranger is pure clown and to me he looks like he's smiling 24/7, nothing gets him down, even while training a stern correction is nothing, very solid boy. I love all 3 of them, no real favorites, I enjoy each ones uniqueness.
4. You will just have to work this out between yourselves. DW and I talked for almost a year before getting Tuke and Zoey. Ranger came to us from an agreement we made with my MIL. I would have liked to have waited until the girls were about 4 before adding another, but my mil needed to have back surgery followed by hernia surgery so she was unable to care for him. We got him at 4 months and he fit in perfectly.