Can I use this bar to mount a guidescope on the telescope? Or do I need a different mounting plate?

This dovetail bar can indeed be used to mount a guidescope on the top of the Celestron 14. What you would need to get in addition with this would be a set of guidescope rings from Losmandy, ADM, or Farpoint that mount onto D-Series dovetail plates. There

According to the description,  the Celestron Nexstar GT Computerized Mount will accept V series dovetails.  Does this mean the Vixen style dovetails?  So I will be able to attach telescopes with standard Vixen dovetails to this? 

Hello Sarkikos, That is indeed correct. You can attach telescopes with Vixen style dovetail rails. You just need to keep under the maximum weight capacity of the mount. If you have a telescope that exceeds the maximum weight capacity of this mount (Which