Hi Singsongster,
the Stearns & Foster coils in the ultra plush and the ultra firm (and everything in between) are the same, it’s just that the layers over the coils are different.
A latex core can be just as supportive or even more so than an innerspring. Once again though the difference is the layers that are above the support layers or components of a mattress.
In general a mattress has two types of “support”. what I call “primary support” comes from the firmer layers in the lower part of the mattress. This is the part that “stops” the heavier parts of the body from sinking down too far and causing the spine or joints to be out of alignment. The upper layers of the mattress “allow” the pressure points to sink in enough in all the sleeping positions to re-distribute pressure away from the pressure points but they also provide secondary support by filling in the gaps in the sleeping profile and helping to support the more recessed curves of the spine.
When you are testing a mattress as a base layer for a topper it will clearly be too firm for comfort but if you know the type of material you prefer in the comfort layers (generally either fast response materials like latex or slow response materials like memory foam) then it’s just a matter of choosing a topper in the thickness and softness that best matches your needs and preferences.
If the mattress needs just a “touch to a little” extra softness or a softer “surface feel” … then a 1" topper is usually enough.
If you need a little to a fair bit of extra softness and pressure relief … then a 2" topper would be in the average range.
If you need a fair bit to a lot of extra softness then a 3" topper would probably be the best choice.
Heavier weights generally do better with firmer and thicker toppers.
Lighter weights generally do best with softer and thinner toppers.
In general … 19 - 24 ILD in latex would be considered to be in the soft range for “average” body types while very light body types may be OK with an ultra soft 14 ILD and heavier body types would probably do better with 28 ILD and above.
Most people of “average” weight would likely prefer the softer “feel” and faster response of a 4 lb less temperature sensitive memory foam vs a denser 5 lb memory foam and heavier body types may prefer the firmer “feel” of many 5 lb memory foams. Memory foam can vary widely in its properties though (see post #8 here and post #9 here) so it would be a good idea to talk with the retailer or manufacturer you are considering to advise you what they have been most successful with in the specific memory foam they supply.
Side sleepers with a bigger differential between their waist and shoulders (usually men) or between their waist and hips (usually women) may need thicker and softer upper layers than body types that are “straighter” to allow them to sink in a little more deeply in the wider areas and “fill in” and help provide secondary support for the gaps in their sleeping profile.
Flatter sleeping positions (such as stomach) generally need a thinner/firmer topper.
Curvier sleeping positions (such as side) or more curvy body types generally need a thicker/softer topper. Back sleepers are in the middle.
Those who prefer to sink in less and prefer a firmer surface feel to their mattress than average will tend to choose a little firmer.
Those who like to sink in more and like a softer surface feel than “average” will tend to choose a little softer.
A mattress that has softer support layers or that already has some layers of softer foam in the comfort layers will generally need a thinner topper than a mattress that has a firmer support layer or where there is already less softer foam in the comfort layers. Too much softness or thickness in the upper layers of a sleeping system (the comfort layers and topper combined) can be risky for alignment.
If you choose memory foam it may be OK to go a little thicker than these guidelines because memory foam displaces as well as compresses and you may sink in a little more evenly with a slightly thicker topper but the other side of this is that if you go too thick with memory foam (over 3" or so) there can be significantly greater risk of alignment issues over the course of the night because memory foam properties change with heat and humidity and time so I would stick with 3" maximum in most cases.
These are some general guidelines and while they may not be “perfect” … in combination with a little bit of testing on different materials (fast and slow response) and a sense of whether your preferences tend towards firmer or softer than the average for your weight and sleeping positions you are in a good position to make a suitable choice.
Your circumstances are less than ideal but at least this way you have the best odds of ending up with a sleeping system that is both suitable for your needs and preferences and will also be more durable.
With polyfoam and memory foam … the density of the foam is the most important part of its quality and durability. Both low and higher density foams of these types can be either firm or soft. In other words … with either of these types of foam the density has little to do with how firm it is or how it feels.
With latex … the density is more closely connected with whether it is soft or firm but this is generally measured in ILD rather then density (or in many cases a supplier will just call it soft, medium, firm etc). The most important part of knowing the quality or properties of latex is not density but the type of latex (Dunlop or Talalay) and the blend between synthetic or natural latex. This can range from completely synthetic rubber to completely natural rubber or any blend of the two. You can read a little more about the different types of latex in this article and in post #6 here.
Some of the better sources for toppers I’m aware of are listed in post #4 here.
Some of the other posts that include some thoughts about choosing a topper that may be helpful include post #2 here and post #2 here and post #2 here and post #4 here (which also includes some of the return/exchange policies I’m aware of).
If after sleeping on a firm mattress (to get a sense of how much more softness you may need) and testing various latex comfort layers in local stores you are still completely uncertain … then buying your topper from a retailer that offers a good return policy so you can test a particular topper with less risk can be a good idea.
Phoenix