I was not addressing the mercenaries either, just that a lower tech base can take and use incremental tech improved equipment from a more advanced tech base. Like the humans in that story, they can graft it on to their existing base even if they do not understand all of it. Some can be replicated on a small scale but other pieces of the technology that required a revolutionary change will not- to use the Worlds at War example, the humans could not duplicate the circuit boards of the radar but could create their own display screens and I think copy the form of the magnetron. Go back to that NAIS building Clan ER lasers, the whole of the AFFC lacks the industrial ability to produce the lasers- tolerances on automated machining, precision of measuring tools, mass production of alloys/composites/polymers of the proper quality, and more- but a collection of scientists and highly skilled engineers can build one by creating the parts and assembling by hand.
Mass producing a more advanced technology requires the pyramid, you have to know how to build the machines that build the parts & machines you will need to create the final product. So IMO if you are building primitive designs, and your engineer/tech force is trained to operate on them . . . if you can get standard battlemech armor imported, then you could replace the standard armor on your existing designs and incorporate it into building new units but producing your own standard armor in any sort of mass produced amount will be difficult. IE, the TC COULD have put that FF armor on mechs after discovering it but they would have wasted a lot of it and not really learned much.