Due to the rather extensive reskining and interlinking across windows for various features, your best bet wuold be to install the entire Fetish set, then patch your pieces over it.
Before you patch in your pieces, I suggest you first try the set as is to see if you can avoid replacing some windows. Once you've identified pieces you want to replace, make sure there isn't some Fetish feature in those pieces that depends on interoperability, then replace the piece with your version.
Off the top of my head, the following features depend on code in other windows.
ClickSearch: buttons in the Examine windows link to the Market window.
Personabar: shown by the Clock window.
ClickStrike: shown by the Clock window.
ClickCures: activated by buttons in both the Player and Group windows.
Hide Any Fog: Depends on the Fetish skin files for EQ2Maps.
You'll also probably want to check the add-ons for the Fetish set to see if there's a version of the piece you want already reskinned for Fetish. For example, there are a few startbutton/command center add-ons that include many of the options available from similar stand-alone mods.
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