Note the large capacity cost on modular limbs. That was intended. Essentially, a modular hand has 4 Capacity in it, but takes 5 off the limb it's attached to. Parial limb same thing, yanking from a full limb. Your overall capacity for a modular limb is, thus, 1 less thana normal limb (The 1 is taken up by the modular joint) ... So, you could have a partial limb with a modular hand, for instance, and have a weapon hand, a shooting hand, a couple of modular bits of equipment, and so on.
For a modular limb that was larger, you could have an armored one, but it won't hold anymore armor than the limb could have held originally. (Less, really, with the reduced capacity).