Hey London locked - when you say piggish - do you mean Pig or just a longboard with the widepoint back (two totally different things) - let us know the model and or dims and we'll have an idea of what to recommend.
My ha'penneth - for a piggie - depends on how you want to ride (all important) if you want to slow the board down then an upright fin with the largest surface area will help to stall you in the pocket, but can be frustrating if you want to down the line speed. Also, you have to generate some foce through the turns to make 'em work properly. I used to use and Almond Huck in my pig, and that worked a treat (i also used a Dee which IMHO was turd for the board - I also ride a Junod and the D in that works grand - but that's pretty much a ful on trad pig)
I tend towards a fin that's upright in profile but with less area, that makes the board whippy and lively through the turns, fast down the line, and a bit of hold to get you noseward. As an all round find i find the Velzy Noserider hard to beat- Also the GC 10 is a corker - a bit more base, more area - like a beefed up velzy.
For the middle ground, an RFC Hatchet (which is a standard pivot and not like a Weber Hatchet) is a grand fin -- as is the Nine Plus Keel (essentially the same fin)
Depends how you like to surf as much as what you surf. Personally I like to ride fast, generate speed, tapping the pocket of the wave as much as possible and going for speed runs when the wave hollows, but I also want that hold for noserides when called for.
As for flexes, they're a mystical beast - but for a log you'd be hard pressed to beat an A-Flex - nice base, flexy tip - hold, whip and noseriding ability - much the same with a Farberow flex - but that might not work so good on a piggie.
It's all horses for courses - nwhat's right and wrong scientifically may not 'feel' right to you. It's all subjective - just play around and have fun chap