Not SB team, but the answer looks pretty easy here
I agree with Bel, even though the 2011 Futura is getting real close to the iSonics in speed, if you like your other two iSonics, stick with them.
If 8m GTX is your biggest sail, and you are about 80kg, the iS117 or iS121 is the best choice. An 8m will work on the iS127/131 but is not ideal as its a bit small. Only leaving you with the iS117/121 for your 8m and 7m. Fin wise, range from 40cm to 45cm (depending on brand/model) for the iS121.
If you plan to get a bigger sail (9x) then the iS131/127 would be even better. Fins 47cm to 52cm.
Also, you say you have iS88 and iS100. Do you mean iS86/87 and iS101?