I think in addition to the size of the launchers, the designs themselves should also be handled differently. Neither the Mk.4 or Mk.5 launchers at 21" can be properly fit into the Fast Scout Cruiser design - at least not as a quad launcher. Triples are possible, but with a degree of hanging off to the side. It's strange out of principle that the torpedo mounts tie their design only to their mark level and not also nations, since at the earliest for custom battles you'll have Atlanta-style ships with Japanese Type 93 launchers.
I think if it could use the Mk.2 launcher - being the closest in design to the Mark 15 launcher - it could properly fit as a quad launcher. This is comparing a quad 21" of Mk.3 (As Mk.2 doesn't have a quad option) and Mk.5 and the length difference is staggering.