I'm 8.5 weeks pregnant with fraternal twins and the er doctor told me i have a small perigestational hemorrhage.. how dangerous is this?

Not very: Most fluid collections noted on sonograms like yours are believed to be incidental + inconsequential even though suspected to be "perigestional or subchorionic hemorrhages". Most women go on to have normal pregnancies and deliveries unless bleeding develops and continues or "placentation" is affected. Your twins represent a greater potential risk which will require careful attention and monitoring