I asked Scotty to comment since he has more experience on the ET and is closer to your weight, but I can say a few things while we wait. I suspect Scott might be sailing some great waves in some remote location...
I've sailed the ET for a few months and prefer the fins a bit forward in the box since I like the looseness this gives. Generally for-aft adjustment works just as with a single fin: backwards for more stability and forwards for more looseness.
As for other fins, I've used both the original fins and MFC 15 and 16 and some custom fins (15). Its a bit hard to say for me at this stage what works best. The board gets looser with the MFC fin, but it also seems to get a bit more unstable when entering the bottom turn. Maybe the MFC fin would work better if positioned a bit further back, but then you would also maybe loose the looseness you gained. The original fins for sure gives excellent upwind and very early planing to the board. With the MFC you need to be a bit more careful with how you push the fins.
With smaller sails like 4.0, I personally do prefer the MFC 15 or 16 though, simply because its less fin. But now I have a 15cm Evil Twin fin I'm gonna try with small sails, but it might not be easy to find that fin where you live. So if you feel you need a smaller fin as s complement, I think you could try either the 15 or 16cm MFC and then maybe position it slightly further back in the box.