I've, lately been playing around with an Empire line dress and various textures for it.  I'd like to have some scenes that include men but most of the men who would have been at the sort of ball such a dress would have been worn to would have been in the military.
Reference material for uniforms of the period I have absolute volumes of so that isn't a problem.  I haven't a clue of even where to start in modeling clothing.  Should I start with learning to do second skins or jump right in?  What program tools do I need?
Any help will be much appreciated.

I would suggest doing texture sets for extisting clothing , with bump, and transparency maps there is clothing that could easily become uniforms .

To make your own , you need a polygon modeller , like wings or blender (both free ) or hexagon, silo, shade , etc , which are commercial products . Since your target application is Poser you dont need the layout or rendering parts , just the modeller .

There is a cheap modeller at Daz called something like Pegasys (?)

That's Pegasus - it's pretty good for the price.

I have some alternative suggestions: PhilC's Clothing Designer is a suite of Python scripts and some props which work within Poser to enable you to assemble basic clothing. You can easily introduce an external modeller into the workflow when you feel the need.

Or if you prefer to jump into modelling, you could try the QuickSuits from my site, which are basic meshes fitted to some of the popular figures. They will give you a head start in modelling since all you need to do is hack them up.