Hi clamothe,
There really isn’t an answer to your question because it depends on your body type, sleeping positions, sensitivities, and preferences and even more importantly how all the layers in the mattress interact together with you in “real life” … not just on the firmness of a single layer. For example if you start with the floor or a firm non flexing foundation as your “base layer” in a sleeping system (which is much firmer than 50 ILD Talalay) and add a mattress on top of it that has a suitable combination of layers you would have a sleeping system that can work well for you regardless of the firmness of the floor or the foundation.
While it’s probably firmer than you “need” … and you may need some extra softness or thickness above it compared to a softer core … with the right combination of layers above it (in terms of thickness and softness/firmness), it could work well.
As you suggested … you could always start with your current 2" latex layer on top of the 6" 50 ILD core and see how it feels for you and then use your actual sleeping experience to help you decide whether you need a transitional layer in between them to isolate you from the firmness of the core a little more (which is quite likely because as a side sleeper you may very likely find that you feel too much of the firmness of the 50 ILD core through a 2" soft comfort layer which could cause pressure points). There isn’t a formula that can predict the layering you will do best with (see mattress firmness/comfort levels in post #2 here) so your own experience and some trial and error and “best judgement” will be the best way to decide on whether you need any additional transition layers once you’ve slept on it.
Once you’ve finalized the combination of layers that work well for you … then you can add a zip cover that is the right thickness to tightly enclose the layers that you end up with to finish the mattress.
Phoenix