Hi Shoulderpain,
It’s not really possible to “diagnose” a cause for your shoulder pain because there are many variables that could be involved including the mattress model you have (there are several estate luxury plush mattresses), your body type, variations of your sleeping positions, your sensitivity to pressure issues, and many others but post #2 here and post #6 here and post #45 here may give you some insights about what may be happening. The odds are good though that your shoulders aren’t sinking in enough to allow your torso to make firmer contact with the mattress and relieve pressure on your shoulders. Your pillow and your mattress protector or anything else you are using on top of the mattress (such as a mattress pad or topper) and even your sheets can have an effect as well.
The Ultimate Dreams 13" gel memory foam is quite similar apparently to the Cloud Luxe except a bit firmer so you could test the Cloud Luxe and see how well it worked for you in any store that carried them to get an idea of how the Ultimate Dreams may work for you as well. Outside of personal testing or sleeping on a mattress there isn’t any way to know or predict with any certainty how well any particular mattress may work for someone but “in theory” the mattress would work well for most side sleepers.
If you did decide to buy the 13" gel memory foam … then you could use your old foundation if it is still in good condition (which I would assume it would be if it’s only a month old). Without a foundation you would need some type of firm and evenly supportive surface for the mattress (such as a platform bed).
Hope this helps
Phoenix