RE: Kombat 86 with very small sails ....
The smallest sail I've used on my Kombat 86 is a 4.2 for my 65 kg. Sorry, never dared to test it with a 3.6 sail or less because it's usually epic conditions and I'm always on my smallest Acid 62 when it's blowing +35 knots...
I'm really surprised how good control I can have with such a "wide" board in strong & gusty wind (25-35 knots; 1.5-2m swell/waves). For my light weight, the 30 cm freeride fin is way too large with such a small sail and makes the board almost unsailable. With the 24 cm wave fin and outboard and back (4) footstraps setting, the board is too stiff and directional but with inboard and front (3) footstraps setup, it's getting much better, more loose and turny.
As for the smallest fin, I can't comment because I didn't test anything smaller than the stock 24 cm wave fin yet, but I guess that a 23 or even a 22 cm fin would be fine with your 3.4 sail.