Well twins have a habit of sliding out if you don't surf them on rail. I'm assuming your fish would be a twin. So that could help your rail game.
A shorter board would probably also be easier to surf more vertical lines. Which also require surfing in rail.
On top of that a shorter board will give you less speed by simply cruising down the line. You would have to manufacture the speed yourself. Now of course getting it on rail is the best way to do that if the waves allow that kind of surfing. (Pumping would also work but that might put you too far out of the pocket).
Downside to a fish compared to a shortboard is that it might allow you to get away with certain things that a shortboard would not allow. But as you're coming from a 7'0 this might be an appropriate step.
Do you have access to the get the right board course? That would help you answer these kind of questions.
Hope that helps!
Commented on -Wetsuit question-Hi friends, I destr...
06 Nov 07:51
Not the UK but similar conditions. I use a hooded 5/4 in winter with 5 mm boots and 5 mm gloves. Most people over here do. I prefer chest zip over back zip. It seems warmer. Have never tried zipperless.