The Imbel is an excellent quality receiver. I had no issues when building an Imbel kit on an Imbel receiver. The para version of the Fal uses a different recoil system and top cover. The receiver is also cut slightly different to allow the use of the para top cover that contains the recoil spring. DSA and others have a top cover out there that will allow you to put a para stock/lower on a standard upper. And yes you can put metric on metric, and even inch on metric and vise versa as long as you use a complete group. For example you can put a inch lower on a metric gun, but all the parts in the lower need to be inch. You can put an inch barrel on a metric gun, but all the gas system needs to be inch and vise versa.