With it's modular design there could be 23rd century upgraded versions of the ship, slightly smoother with the 2265+ style warp engine. So I could easily see later models operating along side the Constitution line.
Yes, and in the back of my mind I've been toying with that idea for a long time. Of course, I can also see how the destroyer/scout design could have evolved from the clipper as well. As systems became less bulky and saucers larger to accommodate more you could easily see that large aft section disappearing, much more compact impulse engines attached to the saucer and the nacelle connected right to the saucer as well via the dorsal.