There are military specs for the ar-15. Most aftermarket parts are made to military specs due to the wide market for them and should fit ARs from most manufacturers. For example: there are only a few receiver manufacturers who stamp them with logos and features for the different AR companies. If the manufacturing process was not held to some sort of standard, their products would be in limited demand. The standard is military specifications. With that said, I have seen some products that needed a little "adjustment" to fit properly.
AR-10 products are another matter. There is no Mil-spec standards for this adaptation of the AR-15 platform so manufacturers have no exact standards to strive for. Therefore it is best to purchase the major components from the same manufacturer.