You should really ask your doctor, or whoever prescribed the Metronidazole. Without being given informed advice, the only safe route is to avoid combining powerful drugs such as these.
Yes you can take metronidazole if allergic to Erythromycin. The drugs that you should be concerned with are Zithromax (Azithromycin), and Biaxin (Clarithromycin) because they are also macrolide...