LOL, I read the WHOLE thing. I'm supposed to be doing income tax, it was a good break.
First off, I must say, you seem to be very lucky in the trainers and foster people you are dealing with. I find their advice to you to be good, since it agrees with my view.
Your dog sounds to me to be well rounded and not a problem. He doesn't like rude upstarts he doesn't know running up into his face, lots of mature dogs do not like that. He tells them so with a snap and doesn't bite them? He's a good boy. He likes some dogs more than others? Again, many if not most, are the same. As long as he is not aggressively targetting new dogs and is polite, if stand offish, in my books that's ok. Let him be the dog he is. He doesn't have to like every dog.
Your newest foster might have two counts against him. Dogs sometimes are upset by disability in another dog. I've known a perfectly nice Lab we all met to attack a newly met blind in one eye GR. Disability bothers some more than others. Plus that dog, I mean your foster, was intact and some neutered males really don't like intact males. That's more of a problem for you if you meet intact males but if your dog simply ignores them then ok. And, your neutered, disabled foster was trapped in the crate. The crate is not viewed by some dogs as a place of safety but rather as a place they cannot escape from. They don't seem to realize the other dog can't get them all they see is they can't get away.
LOL, I think you are right, it's not a big deal. Yes, maybe you'll have to be more careful in choosing fosters. Maybe stick with the girlie pups. I see no cause for alarm, all sounds like normal maturing to me.