I would suggest you read
Peter White's excellent lighting pages. I know he sells them, but the technical information you have there would answer most of your questions.
Now, to get more specific:
1. Dynamos (whichever you select) produce alternating current. Moreover, the Shimano DH-3N70 and SON produce 0.5 A at all times, with voltage increasing with your speed.
2. The good headlights made for generators have these characteristics:
– Only 3 W (or 2.4 W front, 0.6 W rear).
– Well focused beam, so you get the light only where you need it; more efficient use of the light beam and less blinding of others; a 3 W Lumotec is about as bright/effective on axis than a typical 10 W MR-11 spot light, and a 3 W E-6 is even better.
– Some kind of voltage control to keep the lightbulb from burning at high speed.
Basically, forget using the generator for "backup power" unless you want to design your own electronic controller. However, you could use a SON or Shimano dynohub with an E-6 or Lumotec headlight for most of your ride and use your CygoLite headlight for tricky passages.