Hi ErikOA3,
If you are interested in designing and building your own mattress out of separate components and a separate cover that you purchase individually then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you do decide to go in this direction then I would tend to use a “bottom up” approach (see post #2 here).
The only way to know whether any specific combination of layers will work for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) will be based on your own personal experience but at 7" your mattress may be a little on the thin side. There is more about the effects of thickness in post #14 here.
The cover you are considering is also 10" and would be too thick for the 7" of latex you are considering but I would wait until you have finalized your design before ordering a cover so you can make sure that you order the right thickness.
Phoenix