Low shelf and Rll
Will all mfgs lowshelf receivers work with a RLL? Is there an industry standard?
No, and not really. Colt SP1 used to kinda be a defacto standard, but not many manufacturers conform to those dimensions any longer, including Colt.
Of current production, Bushmaster and DPMS will work as-is. Spike's will not work as-is. I don't have personal experience with any others to tell you.
Any lower which has the same internal dimensions as the Colt SP1, or looser, will work. Most low-shelf lowers these days need to be milled back at the rear of the fire control well, and behind the takedown pin post, in front of the buffer retainer pin.