Hi WildSide,
I would tend to wait a few weeks (30 days is a good idea) for your mattress to break in and for you to adjust to a new sleeping surface before deciding on how you feel about it or whether to keep it or perhaps add a topper. There is more about the break in and adjustment period in post #2 here and there are some suggestions in post #2 here that may be very helpful as well (including the suggestion about the type of mattress protector or mattress pad you are using which can make a significant difference in the feel and performance of your mattress).
The most suitable use for a topper is to add some additional softness and pressure relief to your mattress so if after 30 days or so you decide that you need some additional softness and pressure relief then you can certainly add a topper although there is always some uncertainty involved in choosing the type, thickness, and firmness of a topper would be the best “match” for you in combination with a specific mattress. Tuft & Needle also has a great return policy that allows you to try their mattress with little risk so if you aren’t certain about adding a topper then you can also return it and choose a mattress that is a better “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Phoenix