Hi ramblinsarah,
One of the challenges of adding a topper to a memory foam mattress is that it will put you further away from the memory foam and reduce the amount of heat that reaches it which can make the memory foam underneath firmer. This in turn will affect how the topper feels compared to the same topper used on top of a mattress where the top layers aren’t heat sensitive. The type of mattress underneath a topper will play a significant role in how a topper feels.
Because the underlying mattress is very different from the PLB which are mostly all latex mattresses … a topper won’t make one kind of mattress feel like another but it can be somewhere in between because the top layers of a sleeping system or mattress/topper combination plays a significant role in how a mattress feels.
Post #8 here has some topper guidelines that may be useful to you but I would probably start off in the 2" range and then add more if you find it necessary because you can always add more “softness” to a mattress but it’s much more difficult to make a mattress/topper combination that is too soft any firmer except by replacing the topper.
Because choosing a topper can take some trial and error because there isn’t a “formula” you can use to make a choice … the exchange or return policy of the merchant you buy from may also play a role in your purchase decision in case the choice you make is “less than ideal”.
Phoenix