OK, so I have an arduino uno ethernet which i am fairly clueless with. I am planning to put in two of these http://www.pololu.com/catalog/product/2502 and an adafruit motor shield. Is it doable to run all three at once? I am planning to run 4 big motors and 4 small motors, all 12v. also, will this configuration allow for other things, like sensors, to be added?
I think you are going to run short on pins. Each motor uses three pins. 8 motors mean 24 pins and the Arduino only has 19 pins. And some of them are needed to interface to Ethernet. You may have to use motor controllers with serial interfaces (SPI or I2C) or use an Arduino Mega and an Ethernet shield.
i'm not sure you understand, it's not an ethernet shield, it's an ethernet board http://adafruit.com/products/418?gclid=CO7K0b_xibACFQcFnQodB3adMQ it also has a usb2serial adapter http://arduino.cc/en/Main/USBSerial
Digital pins 10, 11, 12, 13 are used for the Ethernet chip. Pin 4 is used whenever you have a microSD card inserted as well.
Even though it's an Ethernet board, it still uses the pins.