They should be fine. The rule is that a potted plant needs to be able to withstand temperatures two zones lower. What I read on line says that Peppermint Hydrangea is hardy to zone 5. With the added protection of the house, it should work.
The bigger issue may be if they will get enough sun to bloom properly.
BTW, I Googled images of Peppermint Hydrangea. Gorgeous!
