Good to here the previous thread helped, thats what the Forums suppose to do.
Yeah the 6ft x 21" fish will make a big difference to bottom end.
Board width and float help alot in the lite.
My board @ 18.5" wide has very full rails and a wide tail like a fish, but with a very shallow swallow tail, and it trucks in the lite.
I know it is trendy to have a deep cut swallow tail, but you loose alot of tail area, so with the quad fin setup I would go for max tail area, cos the fins will hold you in the turns.
Footstrap position makes heaps of difference in the lite, I would put the rear strap over the forward side fin, then your usual stance from there.
This slightly more forward stance on a lite wind board makes trimming upwind on waves alot easier ( it is easier to ride waves upwind in very underpowered conditions) , and means you can dog upwind off the plan in the lulls.
The only other thing you may want to consider is riding a longboard strapless, have heard some people in the past raving about this, good option for small waves???? but if your going out into head high I would go the fish.
Have seen the naish 5ft5"fish down here, it didnt go as well as my board in the lite, the plastic twin fin with deep swallow tail and heaps of rocker works against it.
Hope this helps