That will work.
This method depends on how much you want to customize. If you want to customize the heck out of it, this method might not be best. You'd probably want to duplicate a seperate skin1 directory. If you want to have basically the same look, with different colors and minor changes, I'd do it this way.
Now just customize /skin1/customer/shirtshop.tpl to look the way you want it and change the css include to include a different css file. Take the default skin1.css for example, change it to shirtshop.css, edit the shirtshop.tpl to include shirtshop.css, edit your shirtshop.css, and make any other changes in shirtshop.tpl
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