It's a game engine bug, actually. The reason you can't customize the Federation Renegades is because they are coded to use off-duty uniforms (i.e., civilian clothes or mercenary outfit). Unfortunately, the tailor contains code that prevents BOFFs from using any uniform type except for "Uniform." This creates a logic loop that prevents the system from committing any changes you make to the Renegade.
Cryptic has actually been taking baby steps towards fixing this, such putting the Romulan off-duty uniforms in the "uniform" category and adding the Mercenary 03 belt to the "Uniform" parts list.
Devs, if you are reading, please please please please write a script to go through the database and add ALL of the Off Duty, Formal Wear, and STF Armor parts to the "Uniform" class. Don't change the unlock criteria or anything, just merge them into the "Uniform" group. This will fix so many problems with the current tailor and further enhance the superb customization experience that's a major selling point of your game.
It's really a bit silly there are all these different costume groups in the first place. It doesn't add anything to the game whatsoever and creates complications, like this.
I think they do it because of concerns about clipping issues. However most of the uniforms have clipping issues anyway. So Cryptic, just let us determine if the uniform still looks good despite clipping