1. yes, a 600-watter will be enough for a 4'x4' vegging area (37.5 watts per square foot) ... try for 50 watts per square foot when flowering ...

2. are you talking length, or diameter ? ... the extra length would restrict a bit more, probably not significantly ... if you're talking diameter, the larger will definitely offer less restriction ...

3. my best cloning results involved darkness the first 48 hours, and low light the next couple days, then full brightness ... a 23-watt should be OK for the first few days, but tha's about it ...

4. exhausting into your home's ductwork, will distribute the odor throughout the house ... yeah, I know it's filtered, but a non-smoker will still smell it after awhile, as it builds up in the ductwork ... vent it outside the house, I think you'll be happier ...