SpyOne,
The only problem with this I see is that if you have two NCC-1700's by accident. One's a small ship (like a destroyer), the other's the USS Constitution (which is a heavy-cruiser), unless computerized read-outs (transponder data and the like) read the full formal registry, which for the USS Constitution would be NCC CH 17-00, and for the smaller destroyer would read NCC DD 17-00
I think you are mixing what I said with what someone else said.
What I was saying was that each design of ship ordered by Starfleet (or whatever) would get a unique sequential prefix number. Thus, all ships that begin NCC-15XX would be of the same class, the 15th class of ships.
There could not be both destroyers and cruisers with the number 1700, since that number would designate a specific ship. 17XX was a cruiser, but 18XX might be a freighter, a destroyer, or a scout. (It turns out it was another cruiser, the Miranda Class.)
The addition I made was this: if they want more than 100 of a given class, they give it a new prefix number. Thus, if they built a whole lot of whatever ship got the 5XX numbers, they might start using the 7XX numbers for it. But only if they hadn't already given the 7XX numbers to something else.
This was intended to explain why, in TNG and DS9, ships of the Excelsior and Miranda classes were shown with some very different registries.
This would mean Excelsiors were given, at least, NCC-20XX, -25XX, -45XX, -139XX, -142XX, -144XX, -149XX, -182XX, -340XX, -389XX, - 405XX, -421XX, -422XX, -427XX, -428XX, -429XX, and -504XX.
Miranda were given NCC-18XX, -19XX, -211XX, and -213XX.
The biggest hiccup this gives in the 1900s, where there are Mirandas, the Soyuz Class (an obvious Miranda variant), and the Constellation Class. The handwavium I use for this is to say that the 1900s were used for a variety of Miranda variants, most of which were either refit to the Miranda Standard or retired, but the Constellation proved such a good design that they standardized it as a class, giving ships of that class NCC-28XX, -30XX, and -38XX.
(All registries taken from the Encyclopedia,)
I'm not saying this is canon, I'm saying it fits most observed facts and produces a desired result. If you desire a different result, feel free to discard it.