- You can choose whether a slot acts as a regular slot or an off-duty one, with only the appropriate options for each. Obviously, it'd be better to have an option to 'convert' slots back-and-forth - perhaps a tab for it? So, in the same way as choosing between 'Lower - Long' or 'Lower - Knee-High', you could choose between 'Uniform' and 'Off-Duty'.
- Have all options in the one slot, so you could choose to mix-and-match regular and off-duty parts. I'm sure this would be a popular choice, because it'd increase the possibilities for players, but I personally think that'd be a bad idea - keep 'uniform' parts for uniforms, and 'off-duty' for non-uniform clothing*.
*that said - one concession I can think of would be combadges. I'd be nice to do the time-honoured Star Trek thing of people putting their badge on over their civvies.
The uniform tops are clearly from uniforms and wouldn't look right if they were mixed in with anything else. Now the bottoms on uniforms are generic looking enough that it's not obvious that they are meant to be part of a uniform.
So I propose that if you use a uniform top you have to have uniform bottoms to go with it, but you can use the lower uniform parts with your off-duty wear. That should give a quick and easy way to get more off-duty variety without having to create a bunch of new models and textures.