I'm new to FAL's but quickly becoming obsessed, I have an Imbel upper with barrel already attached, I want to add a lower. I think I can use inch or metric lower, is that correct? I can get a austrailian lower or imbel lower from friend he wants more for the Imbel. Is there any advantage to one over the other? Should I get the Imbel to match the upper? I believe that lowers can be swapped out on the FAL's with out affecting function(headspacing), is that correct? Sorry if I sound stupid, but I'm very new to this platform. Thanks for any and all info.