1.Yes the pixhawk kit comes with the correct cable to connect the GPS/Compass module
2.Correct you will not need a PPM encoder with teh X8R - There are two “S” ports on the X8R. The one on the top of the receiver is the proprietary bus and the one on the end under the channel outputs is the SBus which is compatible with pixhawk. The three wire connector goes from the X8R to the RC input on the pixhawk. You can also find some info on setting up the tarranis with APM here -
3.You could use telemetry through the RC radio but it requires some modification and hacking (someone has done it here), I would suggest that you use the official telemetry kit as it gives you full control of all the options, like changing way-points in flight etc…
4.Yes again the correct cables are included with Pixhawk/ telemetry kit
5.The included power module includes voltage/current sensing. An airspeed sensor will be released for the pixhawk soon, but you can still fly just fine without it.
6.The APM power module provides power to the pixhawk so unless you are using very power hungry servos you might need to use a UBEC to power them.
7.I would only suggest a female to female servo cable to connect your PPM receiver to the pixhawk - http://www.unmannedtechshop.co.uk/servo-cable-female-to-female-15cm.html
Hope that helps.