vice versa probably wouldn't work as well. But also in that specific case, they're... pretty close?
Star Trek 3 makes it out that the Excelsior is new in a way that makes the TOS crew uncomfortable. They complain about it significantly. If not for the onscreen evidence of the existence of the Curry, I would suggest that Excelsior and TMP parts were not compatible.
In other words, possibly a TOS-style ship could be refit to a TMP one, but a TMP ship could not be refit or combined to an Excelsior-type vessel. Maybe all prior ships built specifically for the Federation Starfleet had compatible modules that to a degree that could be updated to newer designs within some limits, and the Excelsior and the related classes were part of the first time that a new, totally redeveloped set of designs, not only due to transwarp drive, which would be why the first ship of that development would be called "The Great Experiment."
It would work so well for the drama of the movie. The Curry prevents the above paragraph from being true, though, if we are to interpret that it really has TMP nacelles, which is hard to avoid seeing. I'm not as worried about the other DS9TM ships since their use of TMP parts alongside of Excelsior parts is more easily explained as "only looking like TMP modules," and not necessarily being the exact same ones.
Why shouldn't newer nacelles just as easily down-tune to work with older power generation?
that makes more sense to me than older nacelles on a newer ship, and I'd go so far as to suggest that the Freedom and Niagara might be ships that once had an older style of nacelle that have been updated to Galaxy-style nacelles in a refit at some point.
On the outside, sure, but who knows what they were full of on the inside? In the extreme example, you certainly couldn't swap a Constitution and a Ptolemy saucer and expect both ships to still work well, or at all. You'd have a Constitution with a second, underpowered main reactor and less crew and fewer science labs in exchange for redundant cargo space and consumables, and a Ptolemy with no reactor at all.
Swapping the modules from the TOS-era to make ships that are literally in-universe kitbash designs is not something I would see occurring. I would assume that the designs of a saucer, dorsal, nacelles, deflector, and nacelle struts were created in such a way that they did not need to be fully redesigned for every new class. I could see a factory making many nacelles for any class of that era, but not just a bunch of saucers being built without thought to which class of ship they would be used on.
no problem imagining the Centaur or the Raging Queen (with rescaled nacelles) were legitimate turn-of-the-century designs that just weren't built in the same numbers as plain old Excelsiors
I could see the Curry and related classes as ships built in 2295, or maybe 2320, based on the NCC numbers, for example, but not as new designs for 2371.
I'm generally thinking that a warp core/nacelles are a single system and that is something that can't (easily) be adjusted.
It seems that might be the case, since it took a major refit to change out the warp drive systems for the first movie. But that further makes the idea of a literal in-universe kitbash ship less likely. The Excelsior-style hull would have to be built with a TMP warp core in mind (and maybe in the saucer in the case of the Curry??) and as such the idea it was made from parts that were found in the site of a battle in space, or built from generic Excelsior and TMP parts Starfleet had in storage somewhere seems even less likely.
And even if you could hot-swap nacelles, I feel like an older warp core would just flat out of be incompatible with newer nacelles, while a newer warp core could still have backwards compatability.
I go with the easiest possible answer of they just didn't have ALL of the parts needed when building these ships, and they were trying to build them quickly.
The idea of missing parts makes sense, but it still does not explain the fitting an older warp drive into a newer hull that was not made for it. For that to happen, the Excelsior would have to have been designed with both its saucer and secondary hull having the backwards compatibility contingencies for older nacelles and or warp/cores.
Not having enough parts might explain some design choices, but I do not see how it explains the idea of putting the Excelsior saucer in the middle of the secondary hull with the nacelles attached to it (or attached to the secondary hull) to create almost a "wheelhouse rear" look almost akin to a present day container vessel, when Starfleet ships almost without exception have their bridges at the forward of center on the ship.
Since that bridge position does suggest a freighter, maybe that could be the Curry's role. It still does not explain the TMP nacelles on that ship though, unless Starfleet just wanted a newer freighter and did not care about utilizing the new transwarp drive for it, but wanted the Excelsior hull for a cargo pod because it is bigger than some other choices? It seems like a lot of effort to have had to design backwards compatibility into the Excelsior's modules, just to allow a freighter design to use the older nacelles when it could be using new ones! (Taking away the secondary hull as a cargo pod, NCC-42043, NCC-42254 and NCC-42284 all have similar designs except for the nacelles and I could see them in the science vessel/frigate/cargo carrier role filled by the Miranda and Nebula in other eras.)
Any interview I've seen with Dan Curry he seemed very thoughtful and intentional. I am sure that the NCC-42254 and NCC-42284 look the way they do for a reason, I really wish we knew why he decided to combine the parts he did.