the 7,5 will work ok with a fin around 45 - 46 cm on your iSonic 127 out of my view. Itīs a wide board with a wide tail, which needs a longer fin. It doesnīt matter how small the sailsize gets. I just would go on a really short fin on the 127, if I would use the board for teaching intermediates or if the water water would be extremely shallow.
Whatīs your sailing level?
Cheers and hope it helps,
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