I've been doing a bit of research and found this link.
Piggyback rails for C8
http://www.scopestuff.com/ss_pigy1.htm
I also found that the maximum weight you can safely add on to your scope is roughly around 2.7 - 3kg including any standard diagonals and eyepieces. I could be wrong.
When mounting the guide scope try to put it as close to the arm as possible to make it easier on the mount. Another suggestion, go for a RACI 9X50 finder scope as a guide scope. The reason why I say this is because by the time you add eyepieces camera equipment etc, you'll be over your weight the limit and possibly overload the mount. I still like the idea of the Orion ShortTube 80 as a guide scope.