Normally I purchase a collection (specific colorway) plus extras if I really love it. I do buy outside of a collection if I really like it. I have some Moda that is years old that feels exactly like silk so I imagine that is the 75x75. I really love that fabric. Personally, I don't think that a company as big as Moda would be putting fabric out there that wouldn't stand the test of time. They have built their reputation on quality fabrics. Everyone has their personal favorites from manufacturer to manufacturer and with the times such as they are one would think they would continue to strive to put their best out there so they wouldn't lose their customer base.
As for mixing different weights I think we've all been doing that for years and not really realizing it. I think it is probably so minimal most don't even notice it. Crazy quilts by nature have always been made by using different fabrics and textures to create interesting looks. Hankie quilts are the same way and I'm sure their are others.
Now if you can see through them like tissue paper there's a problem but I haven't seen any of that with Moda or other top manufacturers as of yet. For me Kona solids seem to have a much heavier weight than most top quality quilting cottons and people use it all the time mixing it within their quilts.
You can go through any big box store or LQS and feel the difference of many fabrics. It just comes down to personal preference and choice if people are pulling from different lines to make their quilts. I just don't think it really makes a difference unless you are creating a show quilt or one to last through generations. Even those are made with slightly different weights that are so often not discernible.