RE: stype fins
In general with the ST's you will loosen the board by running a smaller fin, rather than larger (with obvious tradeoffs to performance, most noteably in marginal conditions).
With 7.5m you're over the ST115's sail sweetspot size range, and the ST115 certainly won't be at it's loosest when over sailed; plus with adding extra fin to try and match the bigger sail really takes you further from where you want to be - if extra loosness is really the aim..
(Tip: Borrow a 42-44cm - anything fast and raked and try it. you'll probably feel that "keel" like tendency that will be there with anything that big, Tomahawk or not, anything above 40cm is big in this board and definitely doesn't compliment the rest of the boards design/ride..)
In 6.4 conditions, especially in powered 6.4m, then taking a smaller fin than stock 38cm FreeSlalom Swift will definitely help, and yes a Tomahawk (or similar) in 34 or even 32 would be a good choice.
Cheers ~ Ian