Can you layer hydraulic acid serum and the StriVectin super C and retinol serum and which order do you put them on?
I guess you meant a hyaluronic serum? Yes, you can layer it with the StriVectin C & Retinol serum, but there is little added benefit: the Stri-Vectin product already contains hyaluronic acid in the form of Sodium Hyaluronate. In case you'd still like to double on it, I'd apply the StriVectin product first, and the hyaluronic serum second simply because the StriVectin contains more actives, and the loss would be greater if it pills off. Otherwise, they can be applied in any order.
Please always be mindful that the more products you apply, the more likely a skin sensitivity is.