Can I ask Phoenix OS (Lollipop) and Remix OS (Marshmallow) questions here?
We already have questions about both (and a tag for the latter), so the general answer seems to be "Yes". Agreed with Dan that it depends on the question.
I would lean towards saying that anything we allow about Android would be allowed regarding these OSes, but I don't know how far they have strayed from stock Android. At some point it might not make sense to host questions about them on the same site. Any restrictions we have about custom ROMs would also apply — "don't ask about nightly builds", etc.