They use two different kinds of viola to similar effect: "kontra" has three strings and a dead flat bridge, "bracsa" has four strings and a not-quite-flat bridge.

There is a set of three books edited by Beata Salomon and her collaborators that covers melody playing, viola and bass. I have the whole lot if anybody near Edinburgh wants to form a band. There's also a smaller book of band transcriptions by Laszlo Lajtha, but that's more Danubian style (as played in present-day Hungary rather than Romania).