In fairness, DPMS or Armalite compatibility refers more to the magazine compatibility and rough shape of receivers (and barrel nuts) than anything else. DPMS, for example, has made both high tang and low tang receiver variants, and different manufacturers who claim DPMS compatibility might either be high or low tang compatible. You can mix and match between manufacturers of receivers if you know specifically what variant you're building, but it is generally true that it is easiest to simply buy the upper and the lower receivers from the same manufacturer and therefore be guaranteed that they are compatible with each other.
Once you get past the receivers, into the broader set of parts needed to complete the rifle, things get a lot more simple.