Can you combine this with the hydration serum?

Absolutely you can as the hydration serum from Summer Fridays does not contain any actives. Any hyaluronic product would be safe to combine as long as it’s just a hydration product. Both the retinol and the hydration serum do contain a lot of oils though, so you might find it makes you feel a bit sticky. If so, just give the hydration serum time to absorb in and make sure the retinol is applied to dry skin only. Don’t forget your sunscreen :)