low wind or larger board freestyle?
firstly, great idea to start holding those comps.
Now, this is none of my business, but I'd like to comment on the format you're proposing, that is, lightwind freestyle and switching format above such-and-such wind.
May I suggest that you want large board (floater, longboard, whatever) freestyle, as opposed to "light wind" freestyle.
Obviously that guy on the video and others are good enough so that they can freestyle those board up to 12-14 knots - easily. Freestyle on those boards gets to be a bit different in stronger winds, but principles are the same. In the old days on larger Windsurfers, the limit was 15 knots or so. It just made for more violent freestyle, that's all.
Personally - and that's me - I make sure that I stretch the capabilities of my boards and freestyle, rather than switch gear (board or sail) as soon as the wind changes by 1 knot. As a matter of fact, methinks this is what kills the sport for some people - the "need" for numerous gears.
By the way, by catering for a wider range of conditions, you have better chance of holding the event too... I know your wind is guaranteed over there, but this is special. For the other 99% of locations that are less ideal, wind is unpredictable, often offshore, etc. Freestylists have to be able to handle adverse conditions too - which those videos boys can handle easily too it's obvious.
Again, none of this really is my beeswax, my unsollicited 2 cents in.
You'z have a great time comes your event in March!