Hi Quentin,
It would be less costly than 11" of latex and with 6" of latex you would have many of the benefits of an all latex mattress (you can see a comparison of an all latex mattress with a latex polyfoam hybrid in post #2 here) but at a lower cost.
There is really no relevant criteria for the height of a mattress and it’s much more important how well the mattress matches your specific needs and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). If a 6" mattress worked well for you it would be just as good as another mattress that was 16" that worked just as well as long as the materials were high quality and durable. The height really doesn’t matter … only PPP … and the design of the mattress and the types of materials and how well if “fits” your criteria are much more important than the height. If you are looking at an all latex mattress you can see some of the potential benefits of greater thickness in post #14 here.
There are no minimum or maximum ILD’s because this is also relative to each person and their body type, sleeping positions, preferences, physiology, and sensitivities. I would use either your own personal testing on local mattresses that are similar as a guideline (using the mattress testing guidelines) or a more detailed conversation with a manufacturer you are considering who can use their knowledge and experience with their own designs and your input from your testing to help you make the best possible choice (see mattress firmness/comfort levels in post #2 here). They are the “experts” and know more about their own mattresses than anyone else.
Phoenix