Hello
1. You can use FK and IK only if you didn't set IK-Target for IK chain.
2. You can use FK for setting constraints limits but make sure (1.) and disabled rotations in constraint are set to 0.
3. I've seen somewhere in internet example of rigging for muscle bending in carrara - just find it.
And small tip for rigging work-flow :
1-set up bones
2-orient joints
3-set up constraints
4-set up IK
5-attach skeleton .
Can you post your .car file ?