This has a lot of good info:
http://www.rmca.org/Articles/abcess.htmI see a couple of antibiotics in the penicillin family listed in the Rat Health Care book (ampicillin, amoxicillin, Clavamox) that are good for preventing infection from abscesses. Or check RMCA's Web site for the drug chart. My vet always has me give them antibiotics in case of infection. Be sure you get a vet to lance that (if you can't get it to drain on your own).