Sloan was interested in securing the safety of the Federation. Once it was discovered that the Dominion was lead by the Founders, Odo became an obvious target for Sloan. The only Founder that Sloan could get to was Odo. The best chance for infecting them was through Odo. Maybe the odds were long that Odo would ever pass it on to the Founders, but it was the best possible route.
And I don't recall Sloan ever caring or being interested in Sisko, just Odo and Bashir (once his genetic enhancement became "public" knowledge). Bashir would clearly be an incredibly useful asset for Sloan, which was demonstrated in the episode "Inter Arma Enim Silent Leges". Despite the fact that Bashir was trying to work against Sloan the entire time, Sloan's plan came off without a hitch! If that doesn't demonstrate that Sloan doesn't need an entire organization of Section 31 operatives, I don't know what would! Sloan managed to make people do what he wanted and needed without them even wanting to help him. What need for other operatives, then?
The Federation is big right? Sloan can't handle every operation, right?
But, hey, when it comes down to it, yeah, the writers took Section 31 in another direction. I'm just saying the canon would easily support an entirely different backstory for S31, one that would be far more plausible, and would prevent fans and writers from pinning all the sinister and sneaky stuff done in Trek on a shadowy organization that managed to keep itself secret for centuries.
That's the really sad thing about Trek Lit. They took this very interesting organization, which had its pros and cons, and just turned it into this strawman evil organization that's only rivaled by Mass Effect's Cerberus in stupidly evil actions.