XCAS does so by an optional tuning file to be added in, MC by an option menu setting. Not asked about, but XCAS will conflict with MC Integration in the sense that they both try to increase the allowable number of sliders limit.
For whatever reason I can't really remember right now, I use XCAS Full instead, probably because I wanted the XCAS framework for other things and was already fully 'invested' in it. There isn't any point in having both and having both won't work at all, one is a core mod solution, the other is a script mod solution, for exactly the same thing. I've never used XCAS Basic but as I understand it, it's only XCAS Full that conflicts with MC Tattoo. The skin tone mod should not conflict with anything. Although it sounds like it's all the same thing, these are actually several very different questions.