Hi Bharri45,
There would certainly be nothing inherently wrong with adding any topper to any mattress that was too firm for you as long as the topper was a good quality material (for durability) and as long as the combination was a suitable match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). A topper can be a good way to add softness and pressure relief to a mattress that is too firm for you but if a mattress by itself is already a good match for you in terms of PPP or is too soft for you then adding a topper wouldn’t be the best idea because the combination may no longer be suitable for you to sleep on.
You could also replace one of the layers in the 12 in 1 customizable mattress with a 3" latex layer in a suitable firmness and you would have a latex/polyfoam hybrid that used good quality materials and an extra polyfoam layer that you could use as a topper somewhere else. If you use a different layer thickness inside the mattress cover then it may be too loose and could shift.
If you are considering a mattress/topper combination that you haven’t tested in person then I would start with the mattress and then use your sleeping experience to decide on whether the mattress was too firm for you (in any particular configuration) and if it was then you can use your actual experience to decide on the type, thickness and firmness of the topper that would have the best chance of adding the additional softness and pressure relief that you need (see post #2 here) because regardless of the quality of the materials … if a mattress or a mattress/topper combination isn’t a good match for you in terms of PPP and you don’t sleep well on it then no matter what price you paid it would have little value to you.
The topper you are looking at is certainly a good quality material but it’s not organic (only 100% natural according to the description) and only your own sleeping experience can confirm whether it would be a suitable match for you with any specific mattress underneath it because the mattress underneath a topper will have a significant effect on how well the combination works for you.
Phoenix