Thanks for writing. While I've not heard of any studies that prove that rubber mulch is bad for vegetable gardens, I don't feel comfortable using it my garden. I don't care for the chemical smell and I don't like the fact that it doesn't decompose --- so it's always there. Instead, I prefer organic mulch that breaks down and improves my soil. (My favorite mulch is compost, actually.)
As far as your powdery mildew problem goes, make sure your tomatoes are growing in a spot with good air circulation. Powdery mildew loves still, humid air. Also, when you water your plants, do you get the leaves wet? If so, that also encourages the disease. When you do your watering, try to make sure it goes directly into the pot and not on the leaves. That should help.
Neem oil is an organic fungicide that can help prevent the spread of powdery mildew. Look for this product at your local garden center or nursery and follow the directions on the packaging.
I hope this helps. Good luck!
---Justin, Senior Garden Editor, BHG.com