Agree that the rod caps need to stay with the rod they were machined to.
On the weight issue, you also need to consider the weight of the pistons and pins. Match up heavier pistons with lighter rods and see where that takes you for the piston and rod assembly. When considering the weight of the rod to do the balance 100% correctly, the upper part of the rod travels in an up and down motion and should be balanced to the piston, rings and pin as a unit. The bottom of the rod moves in a rotating motion and is not included with the piston weight.
