Thanks for choosing GoPiGo. You can connect one Analog, Digital, i2c and Serial Sensor to GoPiGo. It would be possible to have a Camera mounted on the servo kit and ultrasound sensor to be connected to the GoPiGo. I am not sure of what you mean by the contact sensor, if its a reed switch or touch sensor or if its any analog/digital sensor then it would be possible to connect it with GoPiGo.
You will not need a GrovePi as the GoPiGo has the provision to support the Grove Sensors, however if you need to connect more than one I2C sensor or a Serial Sensor, then you might need a GrovePi too.
We use I2C to communicate between the Raspberry Pi and GoPiGo and we use SPI to upload the codes.
Please let us know if this helps,