Hi gnaw,
I’m not clear on what you mean here but if you mean Arizona Premium Mattress’ Adjustable Ultra Plush their twin size mattresses have the same multiple layers as their larger sizes see here and they also do custom layering. The layering of all their mattresses would be the same for twin sizes as they would for the larger sizes.
These are all great quality/value choices and assuming that the one you choose would be a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then there are no weak links in any of them in terms of quality and durability. Of course if you can’t test a mattress in person then I would make sure that you have a more detailed conversation with any online manufacturer you are considering so that they can “talk you through” the options they have available so you have some confidence that your initial choices would be “in the range” of a mattress that would be suitable for you (see mattress firmness/comfort levels in post #2 here). The SleepEZ 8" special does allow for layer exchanges.
Again … I would make sure that you have a more detailed conversation with any of these that you are seriously considering so that you are comfortable that they will also be “in the range” of a mattress that is suitable for you but all of these also use high quality materials and none of them have any weak links in their design either.
When you are considering many options I would start by narrowing down your options to only one with each manufacturer or retailer you are considering based on more detailed conversations with each of them and then you can compare all the pros and cons of your finalists to each other after that. Post #13 here also includes more information about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between your final choices.
I would think that this is probably true “on average” for those that didn’t make the most suitable choice with their initial layers (which in itself would be a minority) but it certainly isn’t the rule because there are also some who choose a latex mattress that is too soft. This would be less likely with an online purchase where the softest versions of latex aren’t as common because most online manufacturers are aware of the risks of making a choice that is too soft. Latex is different from other types of foam materials and has an unusual combination of softness and contouring when it’s initially compressed and support as you sink into it more deeply so there are certainly people where this can “translate” into a mattress that feels firmer initially than they expected (if they are used to polyfoam) at least until they adjust to a different type of mattress.
I would suggest avoiding using specs to compare mattresses or different types of latex unless you have specific reference points that you have tested locally with similar mattresses that use the same type of latex as you are considering because they can mislead you more than help you. For example different types of latex (that use a different method of production or have a different blend of synthetic and natural rubber) will have a different density for the same firmness level. Another example is that ILD is not a good way to compare the firmness of different types of latex because ILD ratings can vary between different types and blends of latex and a layer that has a certain ILD may feel softer or firmer than another layer of the same thickness that is a different type of latex and has the same ILD (see post #6 here). There are also several specs that can affect how soft or firm a mattress or a layer feels for any particular person (see post #4 here). Trying to use specs such as ILD alone to choose a mattress can be very misleading and trying to piece together all the possible variables without having personal experience with how they actually “feel” in real life can be very confusing and lead to “information overwhelm” or “paralysis by analysis”.
Whether you need higher density in any particular layer would depend on which type of latex you are considering and on the overall configuration and design of the mattress. I would keep in mind that density is a “comfort spec” with latex and not a “quality spec”. I would avoid trying to design your mattress ahead of time and work with each manufacturer so they can “talk you through” the options they have available and help you decide which of their options would have the best chance of being a good match for you in terms that will make much more sense to you than overly technical information that can have little practical benefit.
Post #3 here has more information that may be helpful for those that are in higher weight ranges and who may benefit from thicker mattresses or more layers but you are not in a particularly heavy weight range. In general terms though I would tend to work with those that already know what you would otherwise need to learn because “finding an expert” is much easier than going through the time and the learning curve that it would take to become one 
Celsion is now called Talalay GL but it’s the same material. It can have a “slight” effect on temperature regulation compared to “regular” Talalay latex which is already among the most breathable of all the foam materials (see post #5 here). There is also more about all the variables that can affect the sleeping temperature of a mattress in post #2 here.
Phoenix