Cool! Thanks for the advice.
I chose to call it a sinfonia because, well, I didn't know what else to call it. I wrote the piece before I came up with a name. I guess I was thinking of the string sinfonias by Johann Christian Bach and Mendelssohn and then I checked a music dictionary and one of the definitions of "sinfonia" was "a small symphony", so that's why I called it a sinfonia. Is there another name for this piece that would be appropriate?
I also didn't realize that the notes for the viola were so professionally ranged. Oops! I play violin and I used Sibelius to compose so all I did was compose the viola part like it was a violin part. Basically, the notes were fine with me unless they didn't turn red to show that they were too high or too low.