I have no experience with the board, but the 58cm fin should be fine. To maximize early planing and upwind capability with the 11.0, the 62cm fin is a good call, but the 58 will give you a much better overall range and you won't sacrifice much with the early planing. Upwind is likely a non issue unless you are racing on upwind/downwind courses.
I have a formula 160 and have raced with sails from 11.0 down to 6.6. (not the norm by any means with the smaller sails, but I am an old guy, weigh 78kg and have raced a few times in 30 knots on the formula board. I was only interested in survival and finishing.
With that said, I drop from my 11.0 to a 9.2, then 8.4, then 7.6. My favorite sail is the 9.2 and is a reasonable jump down from the 11.0. 11.0 to 8.6 as you suggest is OK, but it's a big gap.
I don't have a fin recommendation for the smaller 8 meter sails, but I wouldn't go shopping until you get some water time on the 58cm first to see how it does with the smaller sails.
If you don't have adjustable outhauls on your sails for this board, it would be a wise investment. Fewer trips to the beach for adjustments, less re-rigging, fewer fins, less stress in the gusts and more fun.
I am anxious to here more about this board. I may go that way when I wear out my formula 160.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.