The fact that vessels engage in multi-role assignments outside their core design intent, I would say that it's safe to assume the there are a shit load of ships of various design within the federation. Space is MASSIVE, and the federation engulfs a silly volume of space.
That said- like now, most ships have a "look" that identifies them as ships, or vessels that float. The same goes for Airplanes. There are some very specific exceptions like submarines and rotary wing aircraft. And I can see the design principals in the federation being mostly the same (the bulk of ships are very identifiable- two nacelles, engineering section, command section), with of course those exceptions like our subs and helicopters (let's say the defiant from DS9 as an example).
Opps- gotta go. Hope you get my point.