Marriss Friemanis, the fellow who designs those Geckodrives, is an experienced engineer; he does a good job designing stepper drivers and has done so for some time. I suppose it's possible that 17-frame motors are entirely different in their voltage requirements, but if you want to quibble about the derivation of that formula, I'd suggest you take it up with him; I'm content to follow his advice without calling it insane, or legendary.

Yes, you can run under that max voltage, and you'll get a maximum speed that's a little lower but still acceptable. More powerful motors will handle more current at the same voltage. Basically, the idea is to run as much electricity through the thing as you can without burning it up, although stepper motors can operate comfortably at fairly high temperatures.