The serum itself brings together several active ingredients: a stable vitamin C derivative, sea buckthorn hydrolate with naturally occurring vitamin C, retinyl palmitate, niacinamide, panthenol and vitamin E. Because of this, it is not necessary to layer another high-strength active on top. In fact, doing so can overload the skin, especially when you are just introducing it.
If you want to keep a separate retinol product in your routine, treat the serum as your retinol-containing step for that day and use your other retinol product on a different day. The same approach applies to strong vitamin C serums or AHA/BHA chemical exfoliants: alternate them with this serum rather than applying both in one routine. When alternating, pay attention to how your skin feels; if it becomes tight, flaky or sensitive, reduce the frequency or go back to using only the serum. During daytime use, sunscreen remains important, since vitamin A derivatives and exfoliating acids can make the skin more sensitive to sun.
If you are unsure whether your other products are gentle enough, the simplest course is to use this serum alone for a few weeks before reintroducing additional actives.