Beater, we lightened most all of the inline race engines we built, not just 6 cylinders but 4 cylinders as well. An inline crank has much different balancing rules than a "V" engine does. In a "V" engine, you have a formula that has to be calculated based on all the weights of the individual rotating components including rings, pistons, wrist pins, wrist pin locks if it has them, a percentage of the connecting rods small end weight and total weight(including bolts, nuts and washers) and bearings. Then a bobweight is constructed and bolted to each rod throw of the crank based on this formula you calculated and it is spun in a balancing machine with these weights attached to the crank. Also, if the crank is externally balanced, then the appropriate balancer and flywheel is also affixed to the crank as well as its balanced.
But for an inline engine, the weights of all these rotating components is insignificant to the balance of the crank. The crank is spun in the balance machine independent of anything else being attached to it. As long as the total weight of these parts for each cylinder weighs the same, it doesn't affect the balance of the crank. The counterweight weights cancel out the weight of opposing counterweights to balance itself. In the same way, the piston, rod, wrist pin weights also cancel out the weights of opposing components kinda' like a seesaw.
Now whether there is a magic number as to how much counterweight can be removed before it affects things. I really doubt it since the mass of the counterweights has no relation to the weight of the rotating mass of the rods and pistons and other components to achieve balancing. If you spin the crank in the engine with all the related components attached to it to 7000 RPM, or remove all the rods and pistons and spin it to 7000 RPM again, the crank is still balanced with or without these parts bolted to it.
