Well, Sela did say that they weren't enough to outright conquer Vulcan, but they would be a dangerous presence that would be hard to wipe out. Imagine setting up a 2000 strong terrorist cell in a nation, that's not easy to get rid of. As to the ships blowing up easily, they were old Vulcan freighters and merchant ships that were unshielded. They were depending on subterfuge, not outright invasion.
And, I hate to say it, but Sela's existence is illogical. Yar died with the Armus killing her. But she wasn't killed in the offshoot timeline created by Enterprise C not finishing it's job, but then went back into the "main" timeline, so there were two Yars in to different places at the same time? Nevermind.