I'll vouch for the 39 cm True Ames shallow water weed fin that Roger recommends- I have one myself and use it to good effect on a 70 cm wide board. I imagine it would be ok on an 85 cm wide board, too.
Personally, I think the 34 cm Select Weed fin is going to be too small, but windsurfing is about experimentation, so please go ahead and try it and let us know how it works out.
Roger- Will you be at windfest in the OBX this year? I'm looking forward to it. I'll be bringing a group of newbies, and hope to try a Futura or a Kombat for myself.