Witch fin will works well for a single use on the 12" gun PSH: 9" performance Future fin, 10" classic Future fins, 10"fluid foils FCS or anythink else I like the 2+1 set with a 8"center fins but wouldlike to try somethink else. Don't know what to buy as a single fin

I wish I had a hatchet style fin in my kit, because it's the last main style of fin I need to try....

But in general, if you're going to be running your board as a single (which was my understanding of your post)..... ;

The longer base of the fin, gives you drive, for long point breaks, etc. But it is argued it gives a little less maneuverability (big word that, and hard to spell !).... Hence, the cut-away at the trailing edge base of some fins.... especially for some surf windsurfing fins, where you have drive from the sails, and you are not desperate for drive from the fin....

These guys have a great range....
http://www.trueames.com/surf.html

Greenough 4C - great all-rounders
4A is a point break special
Heritage / hatchet, probably better suited to small waves, and nose riding.
Greenough Stage 6, is basically a 'cut-away' hatchet
Bigger sized Bonzer Fins actually perform well in large surf, as a single fin.... base drive, plus the right amount of rake, and not too much flax / nice and stiff.....

Short answer;
Get $250 in cash, head down to your True Ames Dealer, and grab 3-4 fins, and play to your hearts content !!

If want anything more technical, read up on angle of attack, spinning out, sharp leading edges, and cavitation... thankfully True Ames are on top of all of this, as their sharp leading edges allow you to increase your angle of attack, with spinning out !! Cavitation is exasperated with blunt leading edges, cavitation = spin out.