The Sidi Hydro's are a good choice and great shoe. I don't recall off hand but mine are I think 2 euro sizes bigger than my normal sidi shoes. I only wear a pair of thick boiled wool socks. I stay dry and warm enough. The key with keeping your feet warm, (that being a relative term) is two fold: one don't constrict them, if the shoes are too snug, like you would with a summer shoe, then circulation is diminished, and they get cold; Secondly, wick the moisture: Once your feet start to sweat and get wet...then you are doomed. That is why I wear a thick boiled wool sock. It wicks the moisture, and a lot of it. Plus wool keeps you warm even when it gets wet.

The chemical warmers work great too on those really unbearably cold days.

Any set of the neoprene booties will work, I have them, but I don't use them with my Sidi's. They are just not needed, and I ride down into the single digits (F°)

HTH
zac