I know its been a while since this post, but I thought I'd reply nevertheless.
Maitreya doesn't support any other physics so far, only breasts.
Belleza does butt, too.
Tonic is something I have info about, if they support all physics, should be worth to look at
The new BBL (YENTEMENTS BBL) has butt physics, too.
I am working on physics for sale and wanted to update the breast physics I have with butt physics, but they seem to be very stiff, like your whole butt moves in one piece, which looks weird, imho
Experimenting with it.. we'll see how it will work out.