I don't remember Satan being a circle in of himself. Satan is simply rooted right in the centier of Judecca, the innermost portion of the 9th Circle. The crime that the innermost circle of Hell punishes is treason against one's own lord, and Satan was the first and greatest (lowest?) traitor.
Back on topic, I'd say St. Nick's dragon would be a silver -- wouldn't mind the year-round cold, probably has some 9th level version of Expeditious Retreat (or perhaps some highly specialized Time Stop that only applied to movement?), and as a silver dragon is generally more concerned with lesser mortals than it is with its own kind.