Yeah, its may seem a bit strange with the fin sizing. In fact, even the 66 uses the same 16+11 setup. Only the 76 differs with a 15+12.
First, regarding the back fins, moving them forwards make the board a tiny bit looser and moving them back makes it a tiny bit drivier. Not a huge difference though. I use them in the middle on my boards. Only on the 71 I used it further forwards sometimes.
As for sizing in general, I think the 71 works really well with the original setup in most conditions. But there is a 13cm option for the front fins and with all those boards I think a 13cm kit would for sure open up some tuning possibilities. Personally I don't think its a matter of sail size. I've been out with the 16+11 and 15+11 in some really windy wheather with no problems. Instead the smaller fins tune the feel of the board. You downtune some of the drive both in the bottom turn and top turn and instead get a bit quicker reactions and a softer feel. I think it makes the boards easier to handle when you are really powered up and waves are bigger and faster. Compared to most boards, there is still A LOT of drive even with the 13cm fins.
So my advice is to get a set of 13cm front fins first. If you really feel like fine tuning you might get a set up 15cms too, but the 16 and 15 aren't really THAT different. There are no smaller rear fins available. I know some aftermarket fin brands are working on some though. And technically, you could fit a really small fcs surf fin which I guess is available. But go for the 13cm front fins first.
Good luck with your boards and please report back when you sailed them some more.