RE: S-type 104 ; fin for 7m??
With the 2007 ST104, stock fin is the FreeSlalom Swift 360, which will be a good midrange fin for your weight with 7.0m on the ST104. For 70kg, you should be fine on the ST104 with 7.0m.
You may like to consider an optional fin for tuning with the 6.0m in higher wind conditions.
A fast freeride style fin around 30-32cm* will be a good choice for that,
and will size well alongside the stock 36cm in your quiver for this board.
* some lower aspect ratio freeride fins may require a slightly smaller "mechanical" size; conversely , some higher aspect ratio fins will measure slightly larger for a given "performance" size.
That's particularly common in freeride fins, where the outline/planshape/AR doesn't always follow a single common trend.
(eg : Low AR 28-29cm = Hi AR 30-32cm in "performance size")
Cheers ~ Ian