My sails are 4.5, 5,2 and 6.0 freeride sails. I'm now using a 33 cm freeride fin on from Gun it, but I tried a 30 cm slalom fin before while using my 6.0 which felt great. With that fin this board felt unstoppable in terms of speed (the 33cm feels like it's holding the board back compared to it), but unfortunately it's not for sale over here in Boracay (very limited availability of equipment).

Anyway, since that 30cm fin held my 6.0 sail fine I am wondering, what's actually the recommended fin size for this board considering the sails I use? Anyone got a good idea of that?

I've also got a 24 cm wave/freestyle fin laying around, will that work with my 4.5 perhaps? Or maybe even the 5.2?

why do you call this board a classique ?
in any case ....
it is a low volume narrow board that seems appropriate for the sails you mention
because it is narrow, it stands to reason it would be faster with a more upright fin
this may not be desired if the water is choppy or wavy

you can check sail size versus fin here:
jimbodouglass.blogspot.com/2010/11/updated-windsurf-calculator-online.html
if you are heavier than 170 lbs then go bigger and vice versa

may help people if they know the name of the board, your weight, etc

good luck and stop breaking boards