Regarding the kitbashes, the Maquis/
Intrepid one is probably the easiest to swallow. The Maquis hull is, after all, just a generic box: there's no reason it couldn't be manufactured in any arbitrary scale. Indeed, we already see it appear in two different sizes, in the Maquis ships flown by Lieutenant Ro and Commander Chakotay, respectively.
The company that builds those ships that have the generic box shape for the stern and an iron or upturned spoon for the bow might well have been contracted to participate in building a companion design for the
Intrepid, just like the
Constitution class once had a companion design with a boxy aft hull. That companion design, too, had oversized impulse engines and drooping pylons...
The
Constitution kitbash with the low-mounted nacelles is also fully to scale and as such plausible. The one with the F-14 hull for secondary hull, with two deflectors, suffers from having mismatched engines, while all the
Excelsior-based kitbashes have a conflict between saucer scale and the scale of at least one other component (bridge, nacelles, torpedo arch). But we don't have to think that the
Excelsior saucer only ever came in one size...
Really, I'd accept that F-14-hulled ship as an "in-universe kitbash" but keep the others as normally designed and procured classes.
It's possible the Yeager class as such never really reached full-scale production with but a handful being built overall.
We saw that class about as often as we did the
Intrepid class... That is, if VOY counts as one appearance.
Timo Saloniemi