I have the "special" pin on my 556 Commando. It's actually the takedown pin for the 551, but it fits the 556. Some companies like MFI are making aftermarket copies of it, but you can find the factory version if you google it.
As for the 556 vs 6920, I'd have both and prefer my 6920 for matches just because I am more familiar with it. But, I must say that the trigger on my 556 is far superior to my Colt.