Is it safe to take sulfa drugs if i'm allergic to thimerosal?

Yes: Thimerosal is a mercury based preservative. Sulfas are sulfonamide based antibiotics. The two don't cross react, but thimerosal may be used as a preservative in topical solutions of antibiotics, including sulfas. If a person has an allergic reaction to a compound with both ingredients, then the doctor has the task of deciding which or both caused the reaction. But this would be quite unusual.