Hi jh376! Welcome to the forums.
To answer your last question first, yes. You can return whichever parts and pieces you don't need -- most everything is sold on a component basis, so there's no trouble picking and choosing only what you need/want. You could buy a 12" door, 2 24" drawers, 30" drawer fronts and a 15" cabinet and noone would look sideways.
As for your cabinet...I'm a little confused. You've got a non-IKEA face frame -- do you mean the door or an actual face frame? that you're putting on a new IKEA Akurum 21" frameless cabinet. You bought wire baskets (to put behind the door, I assume) and a drawer (on the top). Maybe it doesn't matter that I don't get it. What you need to know is that the 21" cabinet is 19.5" on the inside, and the drawers are about 1/4" shy of that. The wire baskets are significantly narrower than the drawers because they are mounted out a bit.
Check
this page for some specific measurements. Hope that helps!
Susan