Yup, pivot and rotary limit joints are a good start.
To give your skeleton muscles you just need more constraints. I need to document this properly at some point, but joints have some adjustable parameters that you can use to make them behave very differently from their intended uses. The most powerful are maxBias and maxForce.
maxForce is fairly obvious, it limits the amount of force (or torque) a joint can use to correct itself. If it requires more force to hold it together, it will stretch. If you want to make a muscle for your skeleton, add a gear joint with a gear radius of 1.0 to the two bodies. That will cause the bodies to hold a specific angle in relation to each other, specifically the "phase" angle that you can change at any time using cpGearJointSetPhase(). If you also set the maxForce for the gear joint, it will work to pull the elbow or knee or whatever to the angle you specify, but won't use an unbounded amount of force to do it. I've found that this works very well.
The other parameter I mentioned is maxBias, this sets the rate (velocity or angular velocity) at which the joint can correct itself. Play with maxForce first, but you can use maxBias to prevent fast or unnatural looking movement.
The docs on the wiki mostly cover this: http://code.google.com/p/chipmunk-physi ... Constraint
I do need to add some recipes though.