it's not really like Rukia defeated Arroniero, she just happen to have a slight advantage and took the chance to use it and everything worked out in her favor. Plus, Arroniero, much like some of the other espada got overconfident in their power over people weaker than themselves and now there dead.
and when it comes to Bleach, from what i've read since the very beginning, it doesn't matter if your the stronger or the weaker, Kubo gives his characters a fair chance at winning and losing.
Kommamura could use his sheer will to defeat Tousen out of being his friend and knowing that he's suffered much in his own life. Tousen was probably promised a life that wouldn't end up like the woman he was best friends with. plus, she died horribly and maybe Aizen used the fact that she was a "shinigami" to tug him into his own plans.
i personally think, Tousen will be defeated before Gin, i'm positive about that.