Because I love universe crossovers (or coincides, I guess in this case, would be a better description), I'm loving this idea, and the Ambassador model is indeed wonderful, and really adds to the scene. The problem here occurs with the update.
We all know that the Enterprise-C was destroyed in 2344, right? And the U.S.S. Nebula was probably built in the same decade, but the Enterprise would have likely only encountered her while she was under construction. Not even the Nebula herself would have been active at the time, much less other examples of the class.
I can understand why you would want to use more than one class, though you probably should have looked for something somewhat earlier.