dlb- I was thinking more generally as far as Rococo design is concerned, but if you look specifically at mirrors, for instance, look in Nutting's Furniture Treasury or a book specifically on mirrors, you'll see that the basic vocabulary of these things is a combination of what I would call volutes, fillets, elliptical sections, arcs and ogee curves.
Depending on the size of the mirror glass, these elements are assembled in a modular sort of way in order to create a crest and base for the mirror, usually in a triangular assemblage with the high point in the centers and tapering to the corners. The crests and bases are blended into the vertical sides of the mirror with "ears" that are usually the same size for the crests and bases.
To design your own period style mirror, have a couple of photos of old ones to refer to, lay out some rough guidelines for "pediment" and "base" and link some of the above elements together using the same basic proportions and massing as the old examples. The old ones all have the same elements in common, they're just arranged in different ways. Different cities preferred different styles, as in other furniture forms. Lots of these came over from England, and even some of the labelled Colonial ones were just being imported by the labellers.
The more flamboyant rococo designs seen in ornate carved mirrors, overmantles and girandoles are an even looser assembly of rococo elements as seen in Chippendale, Locke, Thomas Johnson and others. Carvers put these more ornate elements together to suit their own style, combining plates of Aesop's fables, biblical scenes and scrolls, rocks, trees and Oriental elements together to create some wild stuff in the rococo style. See Helena Haywards book on Thomas Johnson for the real research into this design source attribution stuff.-Al