Hi Fran,
You can certainly add a topper to improve pressure relief if a mattress is too firm and it doesn’t have any dips or soft spots but it can sometimes be almost as difficult choosing the best thickness and softness of topper as it can to choose a mattress so can be less risky to choose a mattress that already is as close as possible to what you need and prefer in terms of PPP (Posture and alignment, pressure relief, and Personal preferences) and use a topper as a “backup plan” if the mattress ends up being too firm and you can’t exchange layers. In most cases toppers can’t be returned if you make a less than ideal choice.
There are some guidelines in post #2 here and the posts it links to that can help with making the best possible topper choice based on your sleeping experience on a mattress.
26 ILD would be fairly firm as a comfort layer if you were sleeping directly on it but it’s on the soft side for a latex support core even with Dunlop. Latex support cores can go well over 40 ILD for extra firm support but of course most people would need a softer layer on top of this for pressure relief and comfort. Dunlop will “act” a little firmer than Talalay because it gets firmer more quickly as it compresses so 26 will be a little firmer than Talalay in the same ILD.
Phoenix