Do i have to use T3 for outer walls only OR inner walls as well?

So we've started our base as standstone foundation/walls/ceilings/etc

I've unlocked the reinforced stone walls and black ice walls and going with Black Ice (easier to get resources)

do i just fortify the outside walls with black ice walls OR do i have to do the inner passage walls/etc as well?
Will it matter if everything inside is still sandstone?