It's not uncommon for the first 12" of dirt to be dry and plants still grow fine in nature. The roots are able to perform uptake of nutrients just fine with out the root ball drying out. The Humidity of the grow chamber will keep the root ball from drying out. With NFT you just need to make sure you have good drainage and flow. You don't want the roots to block the flow of nutrients to the other plants back up and over flow the grow chamber making a mess on the floor.

Ebb & flow you water about 15 min every hour to 2 hours or somewhere in-between. Time will very from strain to strain and plant to plant.

Before you start to grow in Hydroponics you may want to pick up a good book for reference so you can tell when you get good sound advice or just crap talk.

That's the hardest part of growing, knowing when to ask, what to ask, and who to ask. Then to know if it's the right advice or just a 14 year old kid playing with mommy's computer and telling you BS.