Will it be hard to learn viola? (beside the alto clef)
Do my violin background will help me with the viola?
I'm afraid after I learn viola, and getting use to it, I'll forget to place
my finger in correct position on violin. So it become out of tune.
Viola has wider distance, doesn't?
I don't play the viola, but, I think that if you practise the viola and violin enough, both at different time, (viola in the morninng, violin in the evning) for example, or you could put a coloured sticker under the strings on your instruments, with a dot where 1st, 2nd and 3rd finger go to help you. Somtimes, just practise your fingering aswell as the peices.
As for the alto clef, thats easy! The alto clef is a 'c' clef, becuase the middle of the clef sighn is on C. So you can work it out from there. The other way, you know the treble and bass clef, well, D on the treble clef is F on the bass clef. So the note on the alto clef is E because it is between the two.
I play viola and mandolin (which has the same strigs as violin e-a-d-g) without any problems. I have been doing that for several years.Early on I did get lazy with my viola, and reading the music was an adjustment. Just make sure you do not neglect reading music for both instruments!
I personally don't play a violin but I'm pretty sure your practice with a violin should help you play the viola and get a good sound. I personally think its an easy instrument to learn. It is a bit bigger so your fingers shall be stretched slightly longer then usual. Intonation is probably going to be the hardest thing to learn since you already have experience with playing a violin.