I would probably allow the mixing of the armors, but I would use a much steeper price penalty than 1.5x. In a region that has advanced to early modern armor, traditional armors would cost anywhere from 10x and up, since they MUST be made-to-fit to function properly. And that's AFTER the players manage to find someone that even knows how to make it. I would probably prefer to have a player in this kind of region that desires old school armor find a suit in an adventure location and have them take it to an armorer that can fix it up and alter it to fit.
In a region that has NOT advanced to early modern armors, I would probably rule that they are just plain not available. Even if some find their way into the region from an external source, someone with WAY MORE MONEY than the players (or perhaps someone with a royal warrant) is going to get it first. A player in such a region wanting some of the new fangled armor would find it much easier and much faster to simply take it from someone that has it (say, an invading force from a more advanced region?)