I was on both bactrim and flagyl after I had my abscess drained with no issues. I have no idea if they are effective for sinus infections. Many people on here can take zithromax successfully, and I know it is used for sinus infections. I can't take it because it gives me diarrhea, but not because of a gut flora issue.
However, I would like to point out that sinus infections are not contagious. Your boyfriend may have had a virus which led to the sinus infection, and you may have also contracted the virus from him and gotten a sinus infeciton as well. Also, I am almost certain I had a sinus infection last spring, and with time and use of a neti pot, it went away. Recent research suggests that antibiotics don't make much difference when it comes to treating sinus infections (see link) so I chose the wait and see approach. As you are on pred, though, I'd still check with your doc to see if you need to go ahead and treat it.
http://www.webmd.com/allergies/antibiotics