Hi LookingNow,
Post #2 here and the topper guidelines it links to should be helpful and it also links to a list of the better online topper sources I’m aware of. I would use your experience on the mattress by itself as a reference point for the thickness and firmness of the topper that you think would work best for you.
I’m not sure where you read this but it’s not correct and there is certainly Talalay latex (made by both Radium and Latex International) where all the rubber is natural rubber and there is no synthetic rubber in the final product (see the first part of post #7 here).
Phoenix