I have done it with Venice twice. I found that if you are going for Venice, it is best to grab an ally of Hungary and Bohemia right off the bat, spam about 18k mercs, fabricate a claim on Austria, and then attack. Bohemia declined to join, but Hungary did. I sacc'd all of my mercs at the beginning destroying Austrian troops and then once the Austrians were dead, I disbanded the mercenaries. This saved my own MP. I fought two wars against Austria before ever taking any Italy required provinces. The good news was that I laughed in the face of Austria's HRE demands while conquering Italy. Hungary and Bohemia proved fairly reliable allies and good deterents for any outside aggression. Austria would always join to support the Italian minors, but it never posed much of a threat after losing half its territory.
However, dismantling Austria may not have been the best decision for the long-term game as the Ottomans are at my doorstep, Hungary is almost gone, and Bohemia hates me now for some reason. The ottomans hate me but have not attacked yet. I fear if they do, it could get messy. Still, not having the big white bad-a$$ to the north always trying to tell me "no no no" everytime I look at the Italian minors made me think this was the way to go. My first game was brutal, as Austria was allied with Castile, who was allied with Portugal and in a PU with Aragon. Portugal was in a PU with England, so every war against an Italian minor involved fighting Austria, England, Castile, Aragon, and several other random countries. Sometimes, I'd win, sometimes I'd lose, but it made for a tough slog to form Italy.