Code compliance with 120/240v outlet
Locking or not, it makes no difference.


Most likely yes you can. Provided the item does not stipulate that it must be on a 15A circuit.
No, you cannot install 14-20R receptacles and only wire some of the terminals, if that's what you mean. 
Re-reading it I think you want to install the 14-20's normally and only use what you need as per each item. This would be OK.