I would go for the 1mm gravel. On AQ we usually recommend 1 to 2mm gravel.
Don't use the 3-10mm gravel at all. Though, if you don't want to waste the investment, seive out the 3 to 5 mm gravel with a cheap basket and mix it with the 1mm gravel.
JBL Aquabasis was quite popular here in Singapore planted tank community a few years back because it was one of the cheapest specialised aquatic substrate fertilisers. I've used it in a number of tanks and they've done well.
I've no experience with Florapol, but had some interest in it some time back. I've always had the impression that it is more then just iron and can be used in place of Aquabasis.
In your case, I would suggest Aquabasis as the bottom layer, followed by a layer of Florapol/gravel (1mm) mix and topped with just gravel (1mm).
I wouldn't worry too much about pulling the FloraPol out, nor depth. The plant roots will find them and Florapol is easy to push back into the gravel.
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