this is probably the best solution to this problem.Finally we are talking about the same thing. Still, i think the better approach for this is to not separate slavery, but separate the living standards for xenophobes and authoritarian. I would even agree that xenophobes have some low level living standard exclusively (aka the pens), while autoritarian maybe have a special living standard above what a xenophobe can offer. Maybe something like luxurious living standards. Would also play well with indetures assets/servants roleplay. Give them a nice appartment, but charge them a lot so they cant escape. So authoritarian slaves would get luxurious living standards, decent living standards and basic living standards, while xenophobes would get decent living standards, basic living standards and pen living standard (or just called "The barn"). I hope my idea is clear.
Xenophobes on the other hand send the miner and farmers in the pens, together with the delicious livestock, but the battle thralls are decent living conditions as well as the entertainers.
That would give way more roleplay options as compared to your suggestion which would take options away and limit slave types to certain ethics.
It makes sense to disable xenophobes from treating xeno-slaves well, but it also makes sense for them to potentially treat them even worse than just an authoritarian enslaver.