jannie, I'm not on SSI, but a lot of people in my group therapy are, so I've been involved in lots of SSI conversations. I think the only regulations about SSI are for how much paid work you can do. And you can work part-time while on SSI, so being able to spend some time doing a job doesn't negate SSI.
Volunteering has much less stress involved than a paid job, so please don't talk yourself out of giving it a try. It will help give you structure to your day, and contributing to the community can also boost your self-esteem.
Good luck.

mj
<font color=green>
If she spins fast enough then maybe the broken pieces of her heart will stay together, but even a gyroscope can't spin forever</font color=green>