Just because they all use the same external appearance doesn't mean that all their capabilities are the same...
I'm not saying for instance, that the Saladin DD has the same TOTAL power output and same number of weapon systems the McArthur CV does, that would be absurd.
While it is true for example that the McArthur CV has a different set of MISSION specific capabilities and more weapons/fighters than say the Saladin DD, if your using the identical impulse, warp engines and weapons, the BASE superstructure, power system and saucer size, the BASE systems and power numbers have to be the same. Specifically, each of these TOS Feds has to have identical ways to handle the same warp engine design whether its got 1, 2, 3 or 4 warp engines.
My comments were in regards to using the same ruler (yardstick) for specing the ships out as they are being modelled.