I think I found the right place.
I put in default replacement clothing, which is mostly okay, but one particular item of regular cc is broken. I can figure out which item broke it, that's not the problem. The problem is I want both.
Can I somehow extract the Maxis resources the outfit needs and attach them just to this outfit? What's gone are the shoes--went from flip flops to skin-tone colored wedges and a triangular piece at the side hems. The rest of the dress looks right.
Or is this way above my head and I should just get a regular cc version of the item I'm using as a replacement and a hider for the original?

This sounds like you downloaded a Default Replacement mesh, is that right? You could extract the original Maxis mesh and turn it into a custom mesh (tutorial in Bodyshop Meshing, simple-ish) and then your Default will not conflict with it. Its not that hard since you won't actually be changing anything with the mesh, just follow all the steps that aren't done in Milkshape.

Yep, exactly. Thanks a bunch, Phaenoh--that sounds easier than I expected--like requiring less SimPE than I thought it would.