Hi Dymnd,
If the mattress is very slightly too firm … then you probably made the best possible choice along the lines of option #2 in my reply to your earlier post …
If you have only had the Optimum for a short time … then it will still be going through the initial softening period and may end up just where you want it to be in a few weeks (as long as the difference is not too uncomfortable in the meantime). You can hasten this by walking on the mattress evenly over the surface (this can help to shorten the initial softening period of a memory foam mattress although for the sake of others reading this … this is not recommended for an innerspring mattress).
If it turns out that it is still too firm after it has gone through the initial softening or break-in period (and you have gone through the initial adjustment period for any new mattress) … then I would suggest a topper in the material that you prefer (which seems to be memory foam). The thinner the better both in terms of temperature and support and an inch or two would generally be enough depending on the degree of change you need in the comfort and pressure relieving qualities of the mattress. The thinner the material … the less it will compromise support … especially on your stomach which needs the thinnest/firmest comfort layer that “works” for you.
Phase change materials like those in the Sealy work best closer to the top of the mattress but are not quite as dependent on proximity as heat conductive materials (like many of the gel memory foams) so while a topper will reduce the phase change effect to some degree, you can also use a more ventilating and open celled memory foam or a gel memory foam to “add back” some of the temperature regulating properties you may lose. Bear in mind too that the type of foam and the ticking/quilting material used in the upper layers of a mattress is only part of what makes a mattress sleep cooler and the other parts of the “sleeping cool” equation (how deep you sink in, mattress protector, sheets, mattress pad/topper, and bedding) will also have an effect in either direction (either warming it up or cooling it down).
So I would first give it enough time for the mattress and you to go through any initial adjustment and then add a topper if necessary.
Some of the better choices for memory foam toppers are in post #4 here. I would tend to choose either a more breathable memory foam or a good quality gel foam (along the lines of the information in post #6 here about cooling down memory foam) both of which (along with a good breathable cover) can be more temperature regulating than memory foams that more closed cell and less breathable.
Phoenix