I was pretty surprised that I was actually able to find a gray flannel double breasted jacket in XS today.
-very little shirt shows
-it has a single vent (this could be a deal killer - would've definitely preferred double)
-fit was good (I wear XS in the regular line's slim fit jackets)
I'm still deciding if I'm going to keep it.
Don't know about the trousers - small sizes usually run out the quickest. You'll be fine with the shirt though - plenty of shirts are still in stock.
Are the measurements for your double breasted blazer the same as on the website?