I also wear a small Castelli though I'm thicker than you


The alpha jacket fits a bit bigger so you can get a base layer on too, the alpha jersey is more fitted. both are a bit less fitted than the gabba


You may want an XS in the Jacket if you won't be using a base layer. The thing comes with its own base layer anyway.


Castelli gets a rep for fitting tight (on American recreational riders anyway) but I find them usually quite generous in the chest and stomach area, and a just bit tight at the shoulders.