I had agood look at the Techart fins last and they have a good collection of slalom fins, and they do stiff, medium and soft to order. like you, there was not much info around on them so I opted to go with another brand instead. But I'm sure the Techart will be good.
If I had too guess which Techart fin to buy (and only guessing here), I probably take a RSL 06 46cm or RSL 03 48cm both soft. You would be best to speak with the guys at Techart to get best advice.
Alternatively, you could go with tried and tested brands like Select or F-hot. Both are very fast and cost the same as Techart fins, but much less than superb Z-fins and Carpenter fins.
On my iS137 for 8.6m and 9.5m I use F-hot 46, 48 and 52 stiff. I about to test a 47cm soft. If I like it I will get a 51cm soft as well. The 47cm and 51cm will replace the 48 and 52.
I'm not trying to put you off the techart fins, some times it is good to take a gamble on other brands