Hi, Jason -
Right On! It would undoubtedly be more expensive to build two of each wheel arrangement rather than four all alike, but if parts were standardized (same size wheels, cylinders, etc.) the only major differences would be in the boilers and frames and maybe length of tenders. Operating pressures (instead of bore & stroke) could be adjusted to accomodate the difference in weight and hence adhesion / usable tractive effort between the two types, to maximize similarity of parts such as cylinders and pistons.
Three of each type (#6,#7 & #8 and #31, #32 & #33) would be even better, as one of each could be in use and one of each on standby / minor maintenance while the third of each was undergoing heavy repairs on a regular basis. With a cycle like that, no one engine would get "used up" too quickly, and they could conceivably all last for several decades if cared for properly.
- Russo de los Locos
p.s. To further spread the wealth among the Historic Preservation community, contract with the Knight Foundry in Sutter Creek, California, for as many of the necessary castings as possible. (See the following link.)