Yes, probiotic supplements are probably a good idea for anyone using oral peroxide. I make my own homemade kefir (fermented milk beverage) for this purpose.
What has caused you to become scared? If I remember correctly you were using the peroxide to try to overcome chronic sinusitis. Did you not find it to be effective?
How are your allergies and chemical sensitivites? I think peroxide is helpful, but I think Liver Flushes are probably the best treatment for this. Peroxide probably won't make a lasting difference if you have a congested liver. I've had a dramatic improvement in my allergies and IBS after my last few flushes, so I think they've made a bigger diference than I previously realized. I've done a total of twelve so far. (eight flushes last year, and four so far this year) Have you tried doing liver flushes?