The rear fins will surely hold up also to jumping. The tabs (that go into the board) are in fact full fibreglass on these fins. Hitting a rock might be something else though and with some bad luck might break the fin off (which might happen with any fin system, of course). The upside is that it is typically very easy to find replacements since this is a normal surf fin.
As for fin choice, unless you make a deal with your dealer, you have to go with the sizes supplied which for the 81 is 16+11. This has always been a dilemma. Either we supply fins and then the buyer get a good well tested setup, but on the other hand those with special needs might end up with a set they dont want. Or we don't supply and many customers might end up geting fins that don't work so well or simply get dissapointed that the boards are not ready to use out of the box. Generally, what you pay for a fin that is delivered with the board is a LOT less than what a similar aftermarket fin cost. But on the slalom boards Starboard this year elected to not deliver them with fins. But I don't think this will happen with wave boards.
In any case, there is a 15cm front fin (original on the Q76) that you might wanna get. And a 13cm has just been released too. So there are options. But based on my own sailing of both the 81 and the other Quads, I think the 16+11 setup suits the board very well. It looks like a lot of fin, but it feel good once on the water.
Do you have some special reason to want a smaller fin setup than 16+11?