Hi arielariel,
As you probably know from reading here … combination stomach side sleeping is a more difficult combination because they are at opposite ends of the range. Side sleepers typically need thicker softer comfort layers to relieve pressure points in the hips and shoulders while stomach sleepers typically need a firmer thinner comfort layers to help prevent the alignment issues that come from sleeping in a “swayback” position. the best option is usually to have a comfort layer that is “just enough” to relieve pressure when you are sleeping on your side but no more so that the risk of alignment issues on your stomach is lessened. Zoning that has a firmer section under the middle of the body can sometimes be helpful as well.
The softening isn’t an “automatic issue” and is more connected with the quality of the materials in the mattress. If there are higher quality foams or materials in the comfort layers then softening isn’t nearly as big an issue regardless of the type of construction of the mattress.
I think that buying a mattress and a topper separately introduces two unknowns and is much more difficult to get right. First you have to buy the mattress and sleep on it and then you need to estimate what the best topper would be based on your experience and best judgement and this can often take some experimenting.
It’s can be a great idea if you test them both at the same time so you know how the combination feels for you and this has several advantages over a mattress that is one piece without separate layers and a zip cover because a separate topper can be replaced without replacing the whole bed. It can also have a different softer feel than the same layer in a mattress cover but it’s not as good an idea IMO if you have to buy them separately because there is a much greater risk of making wrong decisions (unless of course you are able to get a refund on the topper if you make the wrong choice but this is much less common).
My first choice would always be to get as close as you possibly can with a single purchase.
If you do end up buying a mattress that needs a topper (or you are making an exchange and your options are very restricted and this is the best way available to eliminate lower quality foam in the comfort layers) … then post #8 here can provide some guidelines but these depend on sleeping on your mattress for a while first to help you identify how much “change” is required and this can be a fairly subjective process. It would never be my first choice.
Hope this helps.
Phoenix