COM pin should be connected to the power supply.
Yes you can connect microcontroller pin direct to ULN.
If you want to increase collector current you can connect 2 input pin(ULN) together at the same pin at uC and the collector pins (ULN) the same together
---------- Post added at 23:37 ---------- Previous post was at 23:32 ----------
Usually you connect one end of the coils of a stepper motor to the outputs of the driver IC (OUT1-OUT7), right? Usually the other end of the coils is connected to the positive supply, right? Now if you connect the COMMON FREE WHEELING DIODES pin ALSO to the positive power supply, then you have just connected all the built-in free wheeling diodes as they are needed to protect the IC from voltage spikes coming back from the motor coils at switch-off, ok?
So by connecting the COM to the positive supply where all the other ends of the motor coils are also connected, you have simply placed the built-in diodes in parallel with the motor coils and the diodes will protect the IC.