Having never ridden a Sector 9 before, i cant comment on the flexiness, but obviously others here have ridden them and know better.
That being said, at 200lbs, a great board for GS (which i assume your going for with a large board) may be a Yandall Wreckreational Cruiser....you can find it here...
http://chrisyandall.com/
It's a very stiff board, but stable for GS stuff
Pocket Pistols also make a nice line of affordable wood decks, in all shapes and sizes.
Comet as well, and a friend has one that's 36" (?)...but i have to admit...in my opinion the new graphics on them are "less than desirable"...it's hard to argue their performance though, O'Shei just won the Pump Station II and i'd have to assume he was on a Comet.
Skaterbuilt is also coming out with a line os slalom boards that will be worth checking out.
I'm assuming by your post that your new to slalom, or at least to GS, and dont want to invest a ton until you know you like it
With that in mind, Roe doesnt make a woodcore/graphite board long enough for GS.
Foamcores like Ick and Turner will be too pricey
Foam Pistols and Roes are also in the 200.00 range for the deck.
But for your actual question...can you ride a Sector 9 in GS?...yes, if it's not too flexy...Robert Palmer does and he consistently lays down incredible times.
Sector 9 is a high volume retailer and some questions exist on their commitment to the sport....you wont get the personalized service or product that you do from smaller companies, but you wont pay as much either.
The exception being companies like Pocket Pistols and Yandall, who are still smaller than the big groups, but manage to offer personalized service and a line of boards at great prices for entry level purchases.
Chris Linford...thanks for saying why you dont like the Sector 9's.....saying why you dont like them helps out a new skater....saying only that they suck may only turn them off from the sport.
And yeah, lets do that 2X4 race....steel wheels.....Giant Slalom.....we can share a ride in the ambulance on our way to get put back together again..
My apologies for the attitude....