I have been using my flat water/downwind paddle out in the surf and have just tried using a small shaft length in the surf. That said, I actually preferred it to be shorter when on a wave board in the surf. Is it the general case that these two types of paddles should vary as per what you are doing on the water?

Probably a silly question that I should have considered before, but would be interested to know how these vary with others.

yes ..flat water i have my shaft wrist height ,waves head high ,
but then I have a adjustable paddle i use as well as my fixed
and when riding waves on my cruisers ,i have it in the middle .

I'm 8,10 .waves 72,flat water 82,
cruiser in waves 78
.depends how much you sink your boards too.
get a adjustable ,it saved a lot of cutting and glueing specially if you have
lots of different boards.In the waves i use my fixed paddle when riding my 8'7 and 9,0
having a heavy paddle full of water is a pain ,on the cruisers and small waves you don't fall in as much
so thats when the adjustable comes in handy .
hope i helped.