I have both and, to be honest, I almost only use the false warmer to defrost or warm up homemade porridge from these cups. Works great!

It should fit. They have a smaller diameter than a feeding bottle. After that, I'm not sure it's the most practical, but when you're out and about, why not... but at home, the microwave will probably be much better.

Yes 100% the small ones as well as the big ones. Huge advantage for the frozen porridge.