Hi mightynurse,
There are several ways to accommodate this (see the first part of post #2 here) but one of the most common with some of the online latex manufacturers is a design that has split layering where each side of the mattress is built for each person. All online manufacturers are generally experienced with helping a couple with different needs and preferences choose the most appropriate mattress but a split layering makes the process easier.
Terms like “medium” and “soft” are very subjective and relative to each person and only have meaning in the context of what you are comparing them to. If you don’t have an exact or similar frame of reference then they have little meaning by themselves because what is soft for one person can be firm for another. There are also different types of softness and firmness (see post #15 here) and each person may be more sensitive to one type of softness vs another.
As you mentioned … alignment is the goal and different designs may be suitable for one person and not another. There are no “general rules” or theory that applies to any individual or set of circumstances because everyone is different. “Enough” softness in the comfort layers to relieve pressure and “enough” firmness in the support layers for good alignment is the goal and how much is “enough” will vary with body type, sleeping style, health conditions, physiology, and many other factors.
In most cases it’s wiser and less risky to choose a little too firm than too soft. This is partly because it’s usually better in terms of alignment which is arguably the most important function of a mattress and in most cases because it’s also much easier and more effective to make a mattress that is too firm softer with a topper than it is to make a mattress that is to soft any firmer because this would require the replacement of a layer rather than adding something on top of it. Of course with a mattress where you can exchange a layer or layers that are too soft (depending on the symptoms you experience and which layer needs to be exchanged) … choosing too soft is a little less risky because it can be “fixed” without returning the whole mattress.
I would read the information like you would a good book and not “study” it like you would a textbook. Post #2 here has more about making suitable choices but outside of personal testing … it’s always most effective to talk with the people who are the “experts” about their own mattresses and work with them to make your choice than it is to become an expert yourself. They know more about their own specific designs and which options tend to work best with different people and circumstances “on average” than anyone else. “Just enough” information to be able to ask good questions is all you need. Too much and too little information can both lead to less than ideal choices.
Phoenix