Hi Zara Bear,
Your question may be a little bit more complex than you may realize … particularly when it comes to memory foam.
A mattress topper is designed to add comfort and pressure relief to a mattress and help to provide “secondary support” that fills in the gaps in a sleeping profile but the primary support for a mattress comes from the deeper layers of a mattress. There is more about primary and secondary support and their relationship to pressure relief in post #4 here and in post #2 here
Someone that is in a higher weight range would generally need firmer comfort layers (either in the mattress or as an additional topper) because they will sink into firmer materials more deeply than lighter body weights so firmer materials will tend to feel softer to them than they would for people in lower weight ranges. They will also tend to need higher quality/density and more durable toppers that will soften and break down more slowly over the course of time.
The “key” with a sleeping system whether the comfort layers are inside the mattress itself or added to a mattress in the form of toppers is to make sure that the deeper support layers are firm enough to help “stop” the heavier parts of the body from sinking down too far and that the comfort layers on top of this are “just enough” in terms of thickness and softness to to provide good pressure relief in your most pressure prone position (usually side sleeping) but not so thick that they will “allow” the heavier parts of the body to sink down too far before they are “stopped” by the firmer support layers underneath them.
In addition to the thickness and softness of your comfort layers … it’s important that the quality/density of the comfort layers are also suitable for the body type of the person on the mattress because if they are lower quality and less durable materials than they can soften more quickly than higher quality materials which can once again lead to the heavier parts of the body sinking down too far and sleeping out of alignment. With memory foam and polyoam though … higher densities are more durable but the density isn’t related to the firmness of the material because any density of memory foam or polyfoam can be made in a range of firmness.
When you change the thickness or softness of your comfort layers by adding a topper then it’s also not unusual at all that you will also need a different pillow because with softer top layers your shoulders would sink in more which can change the distance between your head and neck and the mattress surface so a pillow that would be suitable for a firmer mattress and would keep your head and neck in neutral alignment may no longer be suitable for the same mattress with an additional topper.
Memory foams or gel memory foams are all used as comfort layers (not support layers) so they would all be in a “relatively” soft range compared to firmer versions of other materials such as latex or polyoam. Having said that … even though they are all “relatively soft” … there are firmer or softer versions of memory foam relative to other types of memory foam but the firmness of memory foam can also change with temperature, humidity, and the length of time it is continuously compressed so with memory foam layers that are too thick where you may be too far away from the firmer support core and you can start off the night in good alignment but wake up in the morning out of alignment because the memory foam has softened over the course of the night (this is called foam creep). The thickness and softness of the comfort layers or toppers under a memory foam topper will also affect how well the topper works for a specific person because every layer of a sleeping system will affect the feel and response of every other layer.
There are an almost infinite number of different memory foams and gel memory foams that can be chemically formulated to be either firmer or softer, more or less temperature sensitive, or have a faster or slower response time regardless of their density (see post #9 here and post #8 here) so the most reliable way to know the specific characteristics of a particular type of memory foam is with a conversation with a knowledgeable retailer or manufacturer that sells it. In many cases gel memory foams will be more “supportive” relative to other types of memory foam because the gel can help with foam creep and some gel memory foams can get firmer faster as you sink into them more deeply than non gel memory foams and in general they will also tend to be less temperature sensitive but again this can vary depending on the chemical formulation and the specifics of the foam.
In “real life” … there are so many unknowns and variables involved that the only way to know with any certainty whether any topper you choose will be a good match for a particular person in combination with their mattress and any other toppers you are using is your own personal testing or experience when you sleep on it so if you can’t test a specific topper on your mattress before you purchase it or you aren’t confident that a specific topper will work well in combination with a particular person and the additional layers in your sleeping system then the return or exchange policy may become a much more important part of a topper purchase just in case the topper doesn’t turn out as well as you hoped for.
If you are committed to a memory foam topper and you find that a particular type of memory foam was too soft then you would either need to choose a firmer memory foam where you would sink in a little less or perhaps a less temperature sensitive memory that won’t soften as much over the course of the night (which can be difficult to assess because of the variables involved) or a thinner memory foam topper which would keep you closer to the firmer support layers in your mattress and won’t allow the heavier parts of the body to sink down as far before they are “stopped”.
Post #2 here and the topper guidelines it links to has more information about choosing a topper that can help you decide on the type, thickness, and firmness of a topper that would have the best possible chance of success but it’s always important to keep in mind that only your own sleeping experience will be able to confirm for certain that a specific topper will work well in combination with your mattress and the other toppers you are using so there will always be a level of uncertainty and risk involved with any topper purchase where you can’t test the specific combination you will be sleeping on in person before a purchase.
Phoenix