Each vibration motor takes 60mA so all 8-9 of them will take ~700mA. I think if you have a good 2A power supply to the grovePi, then you should be able to power them. If the power supply is not good or not capable of supplying more that 1A, then the 5V power would have some noise and may cause the Pi to restart. I think the best test for this would be to start with 4 motors and write a test to turn them on/off every seconds. Run it for a few seconds, if everything is fine, add a motor and repeat the test.
For cables, Seeed has a lot of options here: http://www.seeedstudio.com/depot/s/grove%2520cable.html?search_in_description=0. The longest one I think is half a meter. I have seen a few people getting the sensors working at even 5 meters, but there are a lot of factors. It would be great if you could keep the wires under a meter to reduce problems with signal attenuation. The problem would be even more with I2C sensors. To extend it further, you'll have to cut the wire and add a longer one in the middle.