How thin can I make the wall of a pipe?
The thinnest wall your printer can print is determined by its nozzle size, and will be a little thicker than that nozzle size.

A great challenge when dealing with thin, hollow cylinders is that the cross-section has very little surface area and it can delaminate easily, especially if the tube is long.

You could try printing the tube with a very thick extrusion with, and using only a single perimeter. That would give better gas-tightness and layer adhesion than two, thinner perimeters, but it may turn out too fragile for your application. In that case, you'll need to print additional perimeters. Sticking to thicker extrusion widths would still be beneficial.

At a two centimeter diameter I'd say the single perimeter has a decent chance of working if you handle them gingerly.