Since you already play fiddle you have a head start assuming you play some music by ear.   If you know tunes and have them in your head you can concentrate on the instrument. It is a little more difficult when a person has to learn the instrument and the music.

Some instructors really frown on using the thumb for noting, but I've never had an instructor so I use my thumb and could not play without it. The thumb is most useful when playing chords but can come in handy when single noting. By turning the thumb so the nail is in contact with the string you can get the effect of the wooden noter.

Some players insist that the basic strum should be toward the player but I first played guitar and my basic strum is away. If I were starting over I would probably try to use the inward strum but had been playing too long to change when presented that bit of information.

I would echo the recomendation of the Ritchie and Bryant books. I think every dulcimer player should have the Ritchie book for playing help, history, and as connection with the dulcimer tradition. Larkin's book is the product of her vast teaching experience and is very practical.   Larkin is a gifted teacher as well as a fine player.

There are also dulcimer workshops being taught around the US and in other countries as well. Workshops can reduce the learning curve and also put players in touch with others who share their interest. They also open one's eyes to the possibilities of the instrument.