First of all, let me thank you for this mod. This is something I've wanted for X-com for a long time. The tech progression from large to small makes more sense to me, and getting the heavy laser earlier means its less overshadowed by the plasma weapons.
Have you ever considered adding an alien alloy cost to the lasers? If you consider a progression like 12/8/4/0 for cannon/heavy/rifle/pistol it solves two of the problems mentioned above. Despite receiving the cannons early on, they can't be manufactured and built infinitely, or at least not without also dedicating engineers to producing other materials. On the other hand, the pistol costing only money is a big advantage over the plasma weapons available later on. With advanced craft, power armor, and weapons all competing for scarce supplies - mere money isn't such a concern.