Hi MJ,
There are too many unknowns and variables for anyone to make specific mattress suggestions for someone else based on specs (either yours or a mattress) or theory at a distance (see mattress firmness/comfort levels in post #2 here) but the steps and guidelines in the tutorial post will give you the best chance of making the most suitable choice in any mattress thickness. Which mattress is best for you would depend on the design much more than on just the thickness of the mattress but having said that … and not taking the many other variables into consideration … thinner mattresses will tend to be firmer than thicker mattresses but this is only in the most general of terms because there are so many variables between specific mattresses and the materials and components inside them. There are many thinner mattresses that use good quality materials (and the tutorial also has some guidelines about the quality/durability of different materials) but only your own testing or experience can know whether they are a good match for you in terms of PPP.
If you talk with Dreamfoam they will be able to tell you much more about the differences between their mattresses you are considering than I could so you can “talk through” the options you have available and decide which of their “thinner” mattresses would have the best chance of being a good match for you.
Overall … I would make PPP (Posture and alignment, Pressure relief, and Personal preferences) and choosing the most suitable mattress a bigger priority than the bed because it will have a much bigger effect on how well you sleep than the bed underneath it and IMO the quality of your sleep would be more important than any specific bed. The platform bed you are considering doesn’t leave much room for a mattress where the sleeping surface would be a more normal height because even a relatively thin mattress would end up being higher than most people would prefer.
There is more about the different types of latex allergies in post #2 here. Unless she has a known type 1 latex allergy (and is probably carrying an epipen) then allergies wouldn’t normally be a concern with the type of latex that is used in mattresses and because there is no contact with the latex.
Phoenix