Hi oad8730.
You are welcome! 
Your intuition in trying the soft on top of extra soft, was good as you may be a candidate for a “dominating layer” which is certainly a viable option and a valid design, What it would do is change both the surface feel of the mattress (it would feel slightly firmer or more “crisp” and slightly reduce how far you sink in to the upper layers of the mattress). It is a more “sophisticated design” and more difficult to predict how it may feel for any particular individual (different weights, body types, and sleeping styles will “feel” it differently depending on how far they sink into the top combination of layers) but it’s also a great way to do some “fine tuning” on a mattress to get a surface feel you like but still be able to sink in enough to get the pressure relief you need.
Flexible slat systems add extra variables into the mixture and create more confusion. You would be better off dealing with a flat surface to fine tune. If you really wish to integrate the flexible slat system into your design you would need to only make changes one at a time – and certainly not at the same time as changing latex layers … this can get too confusing as to what causes the new overall effect.
A flexible support system under a mattress can change the feel and response of the mattress compared to a rigid non-flexing support system (which would be a more common choice for a latex mattress) but this can be either detrimental or beneficial depending on which combination (your mattress on a flexible slat support system vs a rigid non-flexing support system) is the best match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). A flexible slat foundation is an “active” part of a sleeping system just like any of the other layers or components in the mattress itself that compresses or flexes under your weight so it can certainly affect the feel and performance of the mattress for better or for worse.
When these types of support systems are used they are typically used under thinner latex mattresses so that the thickness of the mattress doesn’t negate the effect of the flexible slats and these are also very commonly used in Europe under thinner latex mattresses. Some latex mattress manufacturers use these types of support systems as their “default” or actually recommend them. Many flexible slat foundations also have tension adjusters that can be used to create firmer or softer areas under the mattress which can be used to “fine tune” the feel and performance of your sleeping system so that there is firmer support under the heavier parts of the body (like the hips/pelvis) and softer support under the lighter parts of the body (like the shoulders) but again if a mattress is too thick it can reduce or diminish the effect of the adjustments. In some cases, if each side of the support system has flex and there is no flex in the middle center support beam (or if you have two twin XL foundations side to side with the firmer edges in the middle) then you may be able to feel the firmer center support through the mattress. You can see some additional comments about flexible slat systems vs rigid non-flexing foundations in post #2 here and post #2 here and post #13 here.
If a flexible slat system has the structural strength and integrity to hold the weight of a latex mattress and the people sleeping on it and the gaps between the slats are no more than 3" apart or less, the slatted support surface is about 50% of the total surface area, and it has a center support beam with good support to the floor, then it wouldn’t harm the mattress but you will find that there are some manufacturers that aren’t comfortable with anything except a solid non flexing support surface under their latex mattresses and like many things in the industry you won’t find a unanimous consensus of opinion between different manufacturers so it would be important to check on a manufacturer by manufacturer basis to make sure that your support system wouldn’t invalidate the warranty for any latex mattress that you were considering.
As mentioned above the foundation throws more variables (compounded) in the mix and if you plan to modify it than this needs to be taken into account. I would suggest doing the mattress testing on a flat surface to get it as close as you can to the level of support you need and plushness you desire then you can introduce the foundation if you need to fine tune it with either replacing some slats or keeping it as it is.
Tension adjustable slat systems can be useful to help with zoning, but I would do this under the guidance of a manufacturer/retailer that is experienced person with tension adjustability.
Hips will usually sink in more than the shoulders as there is more weight in that area. When sleeping upon side the difference between shoulder and hips is less than when measured standing up (as scapula adducts and upward rotates) I would also make sure that you reevaluate your pillow to make sure that it is providing a decent alignment to keep your cervical/upper thoracic region in a relatively neutral arrangement.
Zoning systems can certainly be useful and well worth considering for people that have more difficulty finding a mattress with the right “balance” between comfort/pressure relief (under the shoulders especially) and support/alignment (under the hips/pelvis especially) or who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoning in this article and in post #11 here and the additional posts it links to but the only way to know whether any specific mattress (zoned or otherwise) will be a good match for you in terms of comfort, firmness, and PPP will be based on careful testing or your own personal experience.
FloBeds ( our Trusted Members here ) for example, have great zoning system but I would use their advice picking out any layers, as this has a little more science behind and can get very complicated. People incorrectly assume that they need to sleep with the spine in total straight alignment like a skeleton, but this is rarely the case.
I hope this gives you enough information to weigh all your options
Phoenix