Most uppers and lowers made today are going to be made to the same dimensions. The days of Colt using different sized pins for their commercial rifles are behind us. You can take the BCG from your Ruger AR-556 and run it in your other AR-15s... As a unit, it will work. What you can't do is run the M16 firing pin in your Ruger, as the larger flange will get hung up in the notched hammer. Why Ruger used this setup when every other manufacturer including Colt who developed it for political reasons, has moved away from it is anyone's guess.

As far as the Ruger upper, it will connect to other lowers... The part that is "non-standard" is the gas block/front sight assembly and the Delta ring assembly. This has caused Ruger owners some headache with certain after market handguard and rail setups. Ruger tried to reinvent the wheel for market differentiation.