Hi NathanJM,
A pillow that is a suitable thickness and firmness will vary with the type and firmness of a mattress because a pillow needs to keep your head and neck in good alignment and the thickness and firmness that does this best will depend on your body type and sleeping style and on how much you sink into the mattress (which will change the distance between your head and neck and the mattress surface that needs to be “filled in”). Outside of choosing a pillow that meets your “needs” and keeps your head and neck in good alignment, there are also many preference choices involved in choosing a pillow (even more than choosing a mattress) because the “feel” of a pillow and the type of material that someone may prefer is a preference choice and different people can have very different preferences about the type of materials and the feel and response of a pillow that they prefer. There is more about choosing a suitable pillow in the pillow topic here.
When you are testing mattresses I would use a pillow that would generally be a good choice for your body type and sleeping style “on average” but if a pillow clearly isn’t working with a particular mattress (and your head is either sagging or is too high) then you can ask the store for one that is either thicker or thinner.
There is more information in post #2 here about the different ways to test or choose a mattress (either locally or online) that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them.
Unless you have a great deal of knowledge and experience with different types of mattress materials and components and their specs and different layering combinations and how they combine together and can translate them into your own “real life” experience that can be unique to you … I would tend to avoid using complex specs or “theory” to try and predict how a mattress will feel or perform for you and focus more on your own actual testing (hopefully using the testing guidelines in step 4 of the tutorial) and/or your personal experience. When you try and choose a mattress based on complex combinations of specs that you may not fully understand how they will actually feel to you then the most common outcome is “information overload” and “paralysis by analysis”. There are many different types of latex with different properties and firmness levels and differences in layer thicknesses that can make a significant difference in which mattress works best for you so one mattress that meets your general criteria may be a good “match” for you while another one that “appears” to have the same or very similar general specs may not be nearly as suitable for you to sleep on.
When you can’t test a mattress in person though then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else without you needing to first go through the learning curve about mattress theory and design and the many complex specs that may be involved.
There is also more about mattress thickness in post #2 here and in post #14 here but I would focus more on whether a mattress is a good match for you in terms of PPP than I would on the thickness of the mattress because there are so many variables involved outside of thickness alone that can make a significant difference in which mattress is the best choice for you. Based on “averages” most people don’t “need” more than about 8" to 9" of latex unless they are in much higher weight ranges (closer to the upper 200’s or higher) but they still may prefer it depending on the specifics of the mattress and their body type, sleeping style, and individual preferences.
The better options or possibilities I’m aware of in and around the Minneapolis area (subject to the quality/value guidelines here) are listed in post #2 here.
Phoenix