First off, let me say that Dante does not put Caesar in Purgatory. Caesar, like all the other virtuous pagans, is in Limbo, which while a pretty nice place is still part of Hell, though the only punishment is the absence of God. (Of course, now that the Pope has done away with Limbo, who knows.) This is probably a language thing; German uses the same word for both Limbo and Purgatory. OTOH, Cato wanders the shores of Purgatory to keep unqualified people out.

Anyway, Dante sent Brutus and Cassius to the absolute bottom of Hell to share Judas' fate as Satan's chew toy, because he was a Ghibelline. Caesar represents the Empire as a positive political entity.

As for Brutus, I think I've commented before that I find him too smooth. There's really nothing there, he's all image. Just about his only redeeming feature is his relationship with Porcia. But then he blows that when he refuses to rush to her side on the Ides when the rumor reaches the Senate that she has died. The guy just wasn't human.