I just installed 12" monkey bars on my '16 SGS. You do not need longer brake/clutch cables to do this. I was able to use my stock just fine.
Here's one of the best videos for the Rushmore bikes. He's also got one for a '12 I believe.
http://youtu.be/sPvOmEvXQRs
As far as wiring goes, it's my understanding you can release the wiring from the fair in and bring it up to the outlet on the bars, but I didn't like that idea. I would highly recommend spending the $35 to buy plug-n-play wire extensions made by Namz.
I don't know what year your sg is but here's a link to the 14+ extensions. Get the 12" for guaranteed ease of wiring.
Just a note... I bought bars, extensions and polyurethane bushings on eBay all for under $400. Good luck man.