Your post has brought up a concern that I think is rather important, at least from my perspective. When you think about all the tolerances between different boards and fins, it can be too challenging, particularly if also working the special screws needed between US and metric sized fins.
The last thing a customer wants are tolerance differences in the board's integral fin boxes. I can say with my boards that there has been great synergy over more than 15 years. Quite frankly, the Water Rat Tuttle boxes are incredibly consistent, and that has made quite a difference integrating different fins and brands over the years. No one wants to start having to change things that you know are right from the past. If one has a cache of fins over time, especially ones that have worked in so many boards, you certainly don't want to start hacking things with abandon.