I think you'd be fine to use just 1 5 volt wire , but I can't help with resistors. I've only ever used light bulbs and have had to apply up to about a 6 amp load (in an effort to increase voltage) on the 5 volt circuit. This was a very old ps though.

Coincidentally I don't do any mods to them anymore because I bought one of those digital power supply testers from newegg. It just plugs in and does the work for you. It also have a digital display on it. It cuts out almost all the work.