RE: Fin Size Isonic 125
It depends on how you rig the Overdrive, but I'd suspect that an 8.5 m2 Overdrive would be a bit too much sail on the Isonic 105.
Should work well on the Isonic 115 and larger (although I never tried larger than 7.5 m2 on the '06 Isonic 115 demo board I had) but I think it's going to be a little too big for the 105.
Give it a try though, and I think you are on the right track but a 38 cm SR6B is going to be quite small for a sail that large. I think your HS 133 44cm will be better, but if the wind comes up and you really get the 8.5 OD downhauled enough to look and sail like a full on race sail, then maybe it will work.
Hope this helps,