Moving my questions here, so as not to clutter IC too much.
1) Does/did Taylor sense the presence of a demon when Gale was lurking around? Even if she didn't narrow down on it?
Yes.
2) I think I'm familiar with James and Gavin well enough to get a feel for their relationships. I'm less sure where Taylor and Riley stand, or some of the others.
If I'm reading their backgrounds right, I'd guess...
Gale - Taylor's probably seen or sensed her before, but probably hasn't put together that the vague and non-threatening demonic presense she sometimes senses is Gale.
Riley - Riley knows Taylor is suprisingly strong and fast, and has taken out vampires one on one, but knows nothing of Slayer lore, and has only recently come around to believing vampires are real. Taylor and James probably think he's the closest thing they've got to a friend with the police, but don't know how far to trust him yet.
As for the NPCs of any note...
Dawn - this isn't the first time Taylor met her, but the last time was when you were training with some of the other Slayers, between the time Faith & Robin found you and the time you moved to San Diego.
Kelly - Riley didn't know her before this case, so no one but Riley has met her before.
Vin the demon snitch (Merle's cousin) - Taylor has met him before, but they don't get along.