Tape the wires to the exterior of the frame. Do not drill.

As Jim said, I wouldn't drill the frame. Shimano do a specific external junction box which attaches in place of the BB cable guide.

The 5-port is the obvious choice to connect 2+ shifters but you can do this with a 3-port and another internal junction box to connect two additional shifters to the 3-port and rest of the system. I'll be doing this to add the 9150 climbing switches to my brake leavers (using synchroshift). You can pick-up 3-port junctions really cheap on ebay.

You'll probably be needing the long external battery mount which uses the bottle cage bosses too.

I was thinking having them order the shorter version, as the frame size is XS. Am I wrong there?

Thanks!

I believe so as I think the battery bolt holes are spaced differently to the bottle cage bolt holes - the battery ones are narrower.

Although you could try one of these http://www.chainreactioncycles.com/...w||pmt||prd|117127UK

(If you go for the newer BM-DN100 mount and get the Bluetooth unit you can run synchroshift and manage the firmware using the phone app)