Dave de Vos
At 0:43 http://www.youtube.com/watch?v=7wV49Q9mpUI
shows a variation of the forward roll from seated position (the front arm going outside instead of inside). (BTW, the instructor in this video is also a member of AikiWeb, Chris Hein)
It changes which part of the shoulder you roll over. You can try and see which way feels the most comfortable for your shoulder.
This is how my teacher teaches forward rolls to beginners, and I definitely recommend it. Like Dave said, don't overdo it. Even just rocking back and forward is enough to get you used to being in contact with the ground.