Bryan,
I think that is a very good point, but it might take 5-10 years (or more) for the foam to shrink any appreciable level. I personally have had good luck using the black silicone to adhere substrate to. That should provide the flexibility to stay adhered to the foam even if it shrinks.
I did small patches by simply drizzling the silicone over the background in rather generous quantities, then taking a hadful of my substrate mix and just pushing it down onto the foa and silicone. If you wiggle it around a bit, the silicone gets spread around, and it all stuck well for me. I used a mix of the coco husk chips (small and medium sized), activated charcoal, and some sphagnum moss.
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Homer W. Faucett III, esq.
Purveyor of Trivialities and Fine Nonsense