When learning a new instrument, it is important to start from the beginning - even if you know the theory stuff. The reason for this is that even though you may know how to read (how is your alto clef reading?) you still need to develop the technique. Playing the "baby" material is essential for developing a proper pedagogical foundation with your new instrument.
For string instruments, the sound is all in the bow, and pretty much every introduction to string pedagogy will start you out playing whole notes on the open strings. Yes, it can be boring, but the goal here is to start developing solid bowing technique; familiarizing yourself with the instrument.
When I did my undergrad, I had to take pedagogy for all of the instruments, which meant reading from a lot of beginner books. What is nice about this is that the more often you do it, the more quickly you can go through a book because you're essentially just learning the kinesthetic technique.
After you have played a particular instrument for awhile (in your case, guitar,) practice can sort of "plateau" out where you've achieved a sufficient level of mastery of basics and now you're focusing on refining everything. Learning a new instrument from scratch can be very cathartic and almost zen in a way; re-establishing and further developing those strong foundational concepts.
Those books are also helpful for introducing you to the repertoire of the instrument as they often contain famous melodies from select pieces. Saying you don't want to play Hot Cross Buns is a little funny for a couple reasons:
1.) You chose viola. Most of the music you play will be inner accompaniment parts.
2.) You want to play Mozart. Most of the music you play will be inner accompaniment parts - quarter notes like Hot Cross Buns.
3.) Apart from that, you'll probably just find viola solos or orchestral passages that are beyond your technical facility.
- Yes, it's boring, but developing technical facility is paramount. Regardless of what instrument you decide to pick up, you'll still have to go over all of the "easy" stuff if you want to develop a proper foundation and sound pleasing.
Hope that helps.