Hello and thank you for your question. I would not cool sculpt on your buttocks. I would recommend doing Emsculpt Neo on your glutes with radio frequency to reduce the area.
CoolSculpting can be effective in reducing areas of unwanted fat, but it will not tighten skin. In my experience, if there is already some loose or sagging skin, this could worsen after CoolSculpting. If there is no sagging or loose skin to begin with, I have not seen sagging skin develop...