To the best of my knowledge, the RigidBodyCombiner isn’t intended for use with physics; rather it’s intended to reduce the number of visual meshes being sent to the graphics card to be rendered. (See this manual page for more detail.)
As such, applying it to physics objects may well indeed break those physics objects.
(While the name is similar to that of the “BulletRigidBodyNode”, I imagine that this simply reflects that both work on non-animated–i.e. rigid–meshes.)
As to your physics objects, let me ask: what is your intended goal in combining them? Perhaps another approach might be found that will help!