What's the correct way of exporting skinned mesh from 3ds Max to Unity?

So I've purchased this pack for a project I'm working on but I absolutely need to change the UVs of each character and I haven't found a way of changing them without first deleting the "Skin" modifier which I suspect could cause problems.

I also get this error when exporting any of the characters which I'm also unsure if it could be troublesome later on when I import in in Unity.

Also on a side note no matter my axis setting when I export my model (Z up or Y up) my model is always wrongly rotated (I have to rotate it -90° on X) when I import it in Unity