Hi all,
I'm dealing with a very similar problem. Right now I'm using a btGImpactMeshShape for the character mesh and the default btSoftBodyCollisionShape for the cloth. I had to use collision clusters to get the collision system to work, but I get jittery collision response (cloth keeps jumping over the deforming surface underneath even if it is not deforming at all). When I tried to use collision detection without collision clusters there was no response at all. The cloth mesh was just falling through the character mesh. I played a bit with the scale but little changed. Given that the character mesh is going to change (deform) during the simulation, is the choice of collision shapes appropiate? I am calling postUpdate every frame so that shouldn't be the problem (actually the jitter appears even if the mesh does not deform).
Finally, I have been having trouble with the collision response when using different meshes. It seems the collision detection and response depend quite much on the tesselation? I am quite new to Bullet, but in general when I make it work for a couple of meshes, if I change one of them I need to tune some parameters to make it work again and break the first one.
Thanks in advance.