I used pea gravel with pavers in the middle in my small HFGH and the cheaper crushed limestone gravel in the Rion. I don't care for the limestone because it's hard on bare feet. I may look to cover it with some kind of mulch after the plants go out for the summer. I'm not concerned with molds and mildews because the Rion has that problem anyway.

I did 4" X 8" pavers on a base that consists of 3" of 5/8 minus gravel plus 2" of sand. It's worked great and drains immediately if I spill. Any time you talk about wood flooring you're inviting algae/mold growth and the potential for slipping into your greenhouse.
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Wooden pallets are untreated wood. Termites will take them out in a single season.

I used 12" pavers to make a 'walkway' though my HFGH, leaving a center bed for future planting in-ground as well as side beds.
Also set the heat sink barrels on 12" pavers. Where the pavers did not fit the space, I pieced with brick and stones.
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One consideration: brick and/or concrete will add more thermal mass than treated lumber.

Nell

Here is a link that might be useful: More about my South Georgia HFGH