New instruments

I just downloaded MUSESCORE and am running through the instructional videos. And trying to use what I learn right away. So far so good, but there are challenges!

I would like to see the mandolin family of instruments added as follows:
Mandolin (same pitches and clef as violin)
Mandola (tenor mandola, so same pitches as viola, but reading G clef an octave down)
Mandocello (same pitches and clef as cello)

The MIDI sounds would probably better be similar to a guitar than to the violin family.

While i have entered these into my first score manually, I haven't figured out yet how to modify the instrument list. I will be writing for this family much of the time.

Thanks