Creating a tool to help with this problem has been one of my back burner projects for a while now. I was never satisfied with just matching free speeds for different motors as there are many different ways to match motors, including matching free speed, stall torque, max power, max efficiency and matching slope. Depending on the application, you may want to choose a different matching condition.
As for the tool itself, I've worked through many of the thought processes mentioned in this thread and have a ~90% complete tool that combines up to 3 motors at any quantity using different operating voltages and fuses and matched by the conditions mentioned above. I then give the user the choice of plotting the torque or speed on the x-axis as well as giving the choice of plotting the super motor.
I've never gotten the workbook to a fully releasable version, which is why I haven't posted it yet, but it seems prudent to post it now. Plus, this conversation has me motivated to finish it through. I just uploaded the latest version to my motor performance white paper:
http://www.chiefdelphi.com/media/papers/2292