I think that Potions masters would keep snakes more than others, due to the usefulness of shed skin and venom as potions ingrediants.  Miss Sarah was introduced primarily as a non-threatening person for Harry to confide in.  As a general pet, I agree that most people wouldn't keep snakes, especially poisonous ones.  She doesn't know everything, far from it, but she is and will be a charmingly narcissistic, fiercely protective presence.   Harry finds animals easier to relate to than people, so I just gave him a pet that could answer back.

Harry is mistrustful of kindness.  He's had so little of it in his life that he just doesn't know how to relate to it.   Severus is trying.  He's not really comfortable with being Harry's guardian, but he's doing his best for Lily.  I would also like to say that he has probably the most experience dealing with kids like Harry out of all the teachers at Hogwarts.  While not all abused children end up in Slytherin, and not all Slytherins are abused kids, I would think that a significant portion of the children in that house come from less than ideal conditions at home.

I kept Harry's signs very basic, because true signing is a language all its own, with sentence structure and all sorts of stuff that I know nothing about.  So I lept it to just single words that wouldn't require an extensive understanding.  It's all true British Sign Language though, which is similar in some ways from American Sign Language, but different in others.  For example, the sign for father is quite different.  So I used a bunch of different British Sign Language sites to help me out.

Thanks for your comments!