Why Rome is needed to form Italy? Historically Italy was formed without Rome and later conquered this city. ( Remember about heroic fight of Papal zuaves). If catholic wants to form Italy but don't want to fight with pope - he should have possiblity to unite Italy without Rome.
Yes, in the 19th century. Prior to that, the Italian unification was seen as impossible without Rome. Napoleon's Kingdom of Italy doesn't really count in my book, as it was merely a puppet. Even during the unification in the 19th century, it was also seen as unlikely without Rome. But they made a compromise. This is why the capital was moved from Piedmonte to Firenze, to signify the intent of getting Rome, which seemed impossible being protected by France.
I'd say, that while unification without Rome happened historically, it was more the exception than the rule. And besides, it happened outside of the timeframe, when things were different.
Also, what that other guy said.