Hi sleepy,
As you have read … thicker/firmer would be the “norm” for someone who was heavier who also was a side sleeper … stomach sleeping has opposing needs though (thinner for better alignment and less risk of sleeping in a swayback position) so in cases like this … “just barely enough” would be the way to go. Zoning can also help with opposing needs. Because all the layers interact with each other … what works best for you will depend on the combination of materials (comfort and support layers) that are in the mattress but these are good guidelines that can be used as a beginning point subject to what your own testing shows is best. As a general guideline as well … if there is a choice between two mattresses that seem to be roughly equivalent … then a little firmer is usually “safer” than softer because the softness can always be improved but it’s much more difficult to “firm up” a mattress that is too soft and doesn’t provide adequate support.
A fairly thin layer of softer material (that you will tend to “go through” more) may help the same mattress work well for both you and your wife. She may “come to rest” more on the upper layers and feel more of the surface softness while you may come to rest more on the middle layers and the slightly deeper (and slightly firmer) softness.
You can do this either on the phone or in response to an email they will send you asking for your preference. This is also the time to let them know you are a member of “The Mattress Underground” so you will receive their free shredded latex pillow with your order.
I would normally recommend at least some local testing to get a better sense of your needs and preferences. I’m not sure exactly where in SE Michigan you are but post #273 here along with post #4 here may be helpful.
Phoenix