I noticed a lot of skinnies here...

I think Castelli has somewhat different fits depending on the range: race fit is almost one size smaller than their regular fit in my experience, so you have to be careful in what you are looking at. Given the info about you, I think a race-fit small may be too short, the shoulder straps will dig in and make you very uncomfortable in the shoulders, it will lift up too much to make the crotch area too tight.