I'm not a team member but I do have a 2014 Flare, albeit in a smaller size than yours, the 101.
My board came with a tiny fin which at first I found hard work – it's not the length I mind but the fact it's quite narrow too so the area is very small (and maybe I'm a bit overweight at the moment, gulp).
So I got a Choco Starfish 22cms fin – which is the one you get with the 111 version anyway – and that made the board better for me on light wind days. Once the wind is solid, I go back to smaller.
The depth of the Slotbox is the same as a US box, and that fact means you should be able to use the same fin length as you'd happily use in a US box.
Slotboxes a seem very strong and I have them in my wave boards with no problems.
That said, I wouldn't go bigger than a 24cms fin in your 111 Flare as driving this board off the fin is not what the Flare is for and you'll lose spinning or turning performance for the sake of only slightly earlier planing.
Any US box fin can be easily adapted to fit a slot box. (Type the key words from that sentence into Google and you'll find a video showing how to adapt any old US fin head to suit a Slotbox).
New Choco Starfish fins come as adaptable to either US or Slotboxes – with the recesses in the sides already there for the slot box screws. You just need to saw off the US box 'lug' with a hacksaw.