I'm surprised that someone hasn't made a 3d printer controller from an Arduino UNO yet. To my knowledge, it should be possible to use some of the analog pins as digital pins. This would allow you to run four steppers (12 pins), three endstops (3 pins), two heaters (2 pins), a fan (1 pin), and two thermistors (2 pins). That would take a total of 20 pins, which the UNO has. Definitely not as powerful as RAMPS, but you could run a printer off it. I've actually thought about designing and making my own 3D printer controller shield for my UNO, but I've been too busy to do it.
As to plugging an UNO into the RAMPS board, I don't think that it would work because some of the stepper pins are too high on the MEGA for the UNO to attach to.
As to plugging an UNO into the RAMPS board, I don't think that it would work because some of the stepper pins are too high on the MEGA for the UNO to attach to.