To me, this would be more of a question about the lowers and what you have for uppers. Do you have a 16"+ barrel for each lower? With my first machinegun (Colt M16), I preferred to shoot shorter barrels, so I had some of my barrels shortened and threaded. I just made sure that I a 16" barrel for the three semi lowers that I owned.
As far as the M16 fire control parts, I eventually got a DIAS for the versatility. I use one M16 selector with the DIAS. All the other lowers have a semi selector. With a semi selector, the lower can only fire one round at a time. If I left a full auto selector in the host, with that selector in the auto position, the hammer can follow the carrier and cause a slam fire. The selector is not really a wear item that I have seen. So having lots of spares is unnecessary.
Scott