Speaking in general, some things should be barred depending on the chosen ethos, yet most things should still be accessible to most ethos.
Here I agree completely that only spiritual governments should be opposed to - and therefore unable to - give AI certain rights. Extremists should even go so far as to being upset about AI pops being granted rights under a caste system, even synths being nothing more than "constructed slaves", mechanical servants in their eyes.
(But that only based on the ideas in the game of what the ethos represent (*), since I could imagine a spiritual people being accepting of the idea of a ghost in the machine.)
(*) I still find it quite difficult to decide, what each ethos really represents, what the devs thoughts about them are.
Xenophile-neutral/indifferent-xenophobe makes the most sense.
Opposing militaristic with pacifistic makes me think militarists for them are warrior clans/Klingons.
Authoritarian and egalitarian seems not to fit to well either, since I could imagine a kingdom (so an empire in game terms), in which all people have equal rights - well, just for the royal family being a bit more equal then ... - also, what does a government look like that is neither/neutral on this axis?
Spiritualists and materialists I guess are to represent religious fanatics and techno-phile atheists, yet as with all others, if you do not go neutral, than you already are halfway extremist on either path.
Simply doubling the values for both steps in any ethos seems a bit simple here. At least a broader differentiation of what each step allows civics-wise and what would trouble pops belonging to each ethos seems required?