2 Questions..need some confusion sollution

[sonomacast]

Hi all, it's been a while, I am about to embark on constructing my 3rd oven, My first was cast in-situ using refactory concrete, Came out well, My second was using house brick a la Russell Jeavons book, Brillliant and quick. Now I want to cast a segmented oven, I am thinking of building a Napoli type oven (lower vault)over a Tuscan type (Higher vault)but I am afraid the segments will struggle to stay up (Less Counterbalance) Any ideas on this hurdle, Also what is a sufficiant thickness of wall when using refactory, my other cast oven was a minimum of 75mm (3") but it looked a little overkill when I'd struck the forms.
Thank's for your time