Disconnecting powered stepper motors is usually a bad idea, and we've added a notice to that effect on our driver pages. By the way, changing connections while power is applied is generally bad even when you're not talking about stepper motors.
I think your reduction of comments to "Knowledge" is the logical step I disagree with; the ideal is not borne in reality. There is no shortage of information out there, and combing through comments on product pages has to be one of the worse ways of acquiring knowledge. Datasheets, web sites, and books by people who know what they are doing will have a much higher density of the "quick lines from experts" you're looking for.
The quick answer is that the torque goes in a straight line from zero at the no-load speed to the stall torque at zero speed, so at around 10% of the stall torque, you can expect 90% of the no-load speed. So, without factoring in any other potential issues such as whether the voltage range is good for your application or if the motor is convenient to mount, the motor you mentioned should be fine the application you described.
I had not heard about the robotics merit badge, and we have not done anything to get ready specifically for it. I looked around a bit and didn't find any details about it; can you provide some specifics of what's involved or links for more information? What would you like to see us do in anticipation of this launch?