Go with a simple 1000 overhead and 4 plants to start. Once you get a successful grow under your belt, then experiment. Everytime make little changes until you've got it tuned perfectly.

6 feet is short but you'll do OK. Veg until they are 12 to 16 inches tall then switch to flower.

I started with a NL from Nirvana, it was pretty forgiving and a good producer. Two plants yielded over half a pound under a 600 watt.

Don't worry so much about the under canopy getting enough light. Trim that stuff off and let the plant produce more on top instead.