I think if you haven’t received a reply in a day or so it would be fairly safe to assume that either your original email or their reply has somehow gone astray (possibly filtered out by a spam filter or sitting in a spam folder) and I would either email them again or better yet call them.
It would depend on the type of mattress you decide to purchase but I would follow the guidelines in the foundation post here. The gaps in the Nature’s Sleep are a little far apart for an all latex mattress but it would be OK for a mattress that had a polyfoam base.
There are also some similar comments about the Nature’s Sleep foundation in post #4 here.
With higher weights especially … more slats, narrower gaps, and stronger wood is better.
Phoenix
PS : edited in response to the edit in the previous post … Spindle’s foundation would be suitable for any of their mattresses.