I'va tried tails with Mo'Bends and while it works it seems to have one problem, the tail's position is at the shoes not at the back, so either explicit compatibility or configurable position would be nice.

Unlikely this will ever be fixed, I've done all I can to add compatibility to body changing mods.
If a mod changes the players model significantly, there is basically nothing I can do.

Easiest way is to look at https://github.com/kihira/Tails/blob/develop/src/main/java/uk/kihira/tails/client/render/LayerPart.java
You can see how I fetch the part and render it, then just follow the methods called