Hi saraht45,
I don’t know the details of the materials that are in your Beautyrest Elegance firm (and Simmons and most major manufacturers don’t disclose much information about the materials they use anyway) and the mattress thatyou use as a base for a topper … and the amount of softer foam that is in it’s upper layers … can affect your choice of topper.
The best way I know to choose a topper is to use your own experience on a mattress as a guideline in combination with the topper guidelines in post #2 here (and especially the posts it links to as well which also include some of the better sources for toppers) which will give you the information and insights that can help you substantially improve your odds of making the best possible topper choice.
Phoenix