I didn't even think about separates before I knew my style type. But how you put separates together actually has a big effect on the impression you make.
Romantics, Ethereals, Classic, and Dramatics are each best in a head-to-toe look. If you think about it, this makes sense, because all four of these types are formal and grown-up in their own way: the Romantic is mature womanly sexiness, the Classic is a "ladylike" adult woman, the Dramatic is a powerful ruler, and the Ethereal is an immortal being. None of these pure types is youthful or casual enough to look her best in an obvious use of separates.
Another easy way to do a head-to-toe look is to just wear a dress. Ethereals, Romantics, and Classics have a lot of great dress options.
Speaking of which: the message a Natural sends with her use of separates is, "I own nothing but separates, and I basically just throw them together because that's how confident I am, but the effect is never weird, because I can't be bothered to put in enough effort to make it deliberately weird, because that's how casual I am."
A Gamine's use of separates says, "I deliberately combine separates no one else would dare to put together, because that's how fun and quirky I am. I want you to notice!"
If you're a blend of two or three types, as most women are, and you manifest Natural, Gamine, or Ingenue through your use of separates, you'll want to lean a bit more heavily on your other essence (or two) to balance the effect. So, for example, A Natural-Classic-Ingenue combining separates in a casual, Natural way would take extra care to bring in Classic and Ingenue in other aspects of her look.
If you're not sure of your style type, try the Style ID Calculator!