Actually, the dominant tuning these days is DAD, a shift from the earlier DAA, but both have there ardent supporters.

Using the thumb is OK, so is not using it. I use mine when I'm doing a lot of chord melody playing, and tend to ignore it when playing fast fiddle tunes.

everythingdulcimer.com is a good website, and I second the recommendation to attend a workshop or festival or get involved with a club. I'm involved with the Pocono Dulcimer Club in Stroudsburg, PA (USA) and we have a one-day festival on January 21st that has a track of workshops for absolute newcomers. More info on the festival is available at Winter Dulcimer Fest   One thing I've learned about dulcimer folks -- we're ardent proselytizers for the instrument, so you should find yourself welcomed with open arms anywhere you go.