You'll definately need bigger than 36 cm for the 9.8. Maybe 46 cm with a regular weed fin, or 40 cm with a Lessacher weed fin. Roger Jackson is an expert on weed fins, so maybe he'll chime in.
With regards to the 50 cm stock fin and a 9.8, I think it will work when you are powered up, but the 58 might help in marginal conditions. It's worth a try, anyway.