With weed fins, you can go a bit smaller as they have signifcantly more are for a given span (depth).
But, if you've sailed your 14.5" Blade Weed on the Futura 122 with a 6.0 and not experienced any performance or control issues, the 14.5 may be fine.
Tailwalking, and control issues usually are not such a factor with weed fins.
Give it a try, and if it feels wrong. or the board acts squirrelly, then look for the smaller fin.
You may not have to go all the way down to 12.5 (32 cm)
Perhaps True Ames, or some other weed fin producer makes a 13.0- 13.5" weed fin.
Tangent makes a 35 cm I pretty sure, and I've had great luck with the Tangent Reapers.
They don't shed all weeds all the time, but the shed most of them, so unless you have tons of floating surface weed (eel grass) the Tangent Reapers really work well. Good speed, pretty good upwind.
Hope this helps,