First,
www.brewboard.com is another great online resource.
Second, lots of people don't even bother with secondary. Since you probably don't care how clear your beer is going to be, I wouldn't bother. Just go straight from primary into the bottling bucket into bottles.
In general, you don't want to move it from primary until fermentation has more or less stopped, which you can tell from your hydrometer readings. It may drop a couple more points in secondary, no big deal. Do NOT add water when you transfer, that's just begging for an infection. The reason that headspace may be a worry is that you don't want to oxidize your beer, since that'll cause long-term stability and flavor problems. It's never a worry in primary because the fermentation is throwing off so much CO2. It should still be throwing some in secondary, so it isn't a huge deal, but I'd always go with the tightest-fitting fermenter I had for secondary. All in all, I'd rather take a chance on an extra couple inches of headspace than watering down my beer and risking an infection ruining the whole batch.