I like the big, thin, triangular picks for the dulcimer. I use a medium, rounded tri-corner for guitar, mandolin or tenor banjo.

I saw a flat pick today with a piece that went around your thumb and 3 or 4 little holes to stop them from falling off - kind of a combination flat/thumb pick. They came in various thicknesses.
It seems like something that GUEST,leeneia would like for her dulcimer. I'll try to get the brand name if she's interested.