RE: Kombat 105
You can use a bigger fin for 7.0/7.5, but don't overfin it as it really doesn't suit the character of the the Kombat - and from a practical viewpoint won't turn it into a super-planer either !
Think about more like 33-34 semi raked freeride, that is "small" for 7.5m by some slalom standards, but not unsuitable for the K105 in freeride mode.
Of course, if you are tuning for lightwind 7.0m wave sailing, the choice would be a (very big) wavefin, which would be a lot lower aspect ratio and so sized more like wave 27-30cm.
Cheers ~ Ian