Hi widey,
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of and a link to the online suppliers that have good exchange/return policies as well
A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
A 1" comfort layer would work well for someone that only wanted “a touch to a little” additional softness and pressure relief but it would be thinner than most people would need on top of a single layer of firm foam (2" - 3" would be a more typical thickness) but each person is unique so a combination that would be unsuitable for some people to sleep on may work perfectly for someone else.
Your strategy makes sense if you are still uncertain about what you are likely to need after talking with a few suppliers on the phone so you can decide to add an additional topper if your actual experience indicates than an inch isn’t enough for you. You can also purchase a dedicated cover once you have found the topper or combination of toppers that works best for you.
Phoenix