Hi connerfamily,
Without personal testing the best way to choose your layers is based on a conversation with the manufacturer who will provide you with suggestions based on the “averages” for people that are similar to you.
The only layering combination I would avoid in the long term would be a soft layer on the bottom. The rest are all possibilities as long as they provide you with good PPP (Pressure relief, Posture and alignment, and Personal preferences). For some people a layering that is medium over soft over firm would work very well … for others it wouldn’t. Normally the soft would stay on top for those who liked the surface softness for better pressure relief and you would move the firm up one level to change the support if that was necessary (or exchange for a firmer layer to replace one of the lower layers). If you wanted to create a firmer sleeping surface then you could put the medium on top and then the firm and the soft on the bottom (temporarily because the bottom layer has the least effect on the surface feel) and if this worked better then you could exchange the soft on the bottom for a medium or a firm. Everything depends on whether you are trying to change the comfort/pressure relief (upper layers) or the support/alignment (deeper layers) and how you want to shift the balance between these two basic functions.
If you test mattresses locally and you find that a 2" softer layer over firmer layers works best for you then the 9000 would be a good choice. If you are a primary stomach sleeper and didn’t sleep on your side or back and thought you wanted a comfort layer that was thinner and closer to the firmer layers (because of the alignment risk of stomach sleeping) then the 9000 may also be a good choice. Of course these are just two possibilities and there is more information about some of the basic “theory” in the mattresses section of the site which has more information about different types of layering and designs but I would tend to just talk with the manufacturer themselves who already knows what you would otherwise have to learn. Your own personal testing and more detailed conversations with SleepEz who know more about their own mattresses and which design “in theory” tends to work best for different types of sleeping styles and body types than anyone else is the best way to answer the types of questions that are about which design or layering combination would work best for you.
Latex is sticky and doesn’t tend to shift or separate once it’s in place (and the cover also fits tightly around the layers) so this isn’t an issue in normal use but if for some reason they do (because you moved the mattress or turned in on its side etc) then it’s easy to unzip the mattress and “wave” the layers back into position.
The most important part of any online purchase is to make sure you talk with any manufacturer you are considering and ask them all the questions that are important to you.
You are in somewhat of a “mattress desert” but the better options or possibilities I’m aware of in your area are listed in post #4 here. A few other lists that are further away are listed in post #5 here.
Phoenix