No one is going to tell you that 2 nights are "enough" for Rome, but you have to do what fits into the time you have. Rome has many, many sights; read a guidebook and see how many of them you want to go inside and can't stand to miss. You can cover a lot of ground in one day if you're basically just wandering around and looking at interesting buildings. I do that a lot. A tourist who wants to really see (go inside) sights like the Coloseum in Rome needs considerably more time. And Rome is large, so simply moving from one place to another can take a good bit of time. Florence is much smaller, but you could potentially spend a lot of time inside museums there.
It will take time to move from Venice to Florence to Rome to Positano and back to Venice.
You need to figure out how much time you'll actually have between arrival and departure in each city. I love the idea of staying in Venice for a good while and absorbing the atmosphere. Consider dropping either Florence or Positano. Dropping Positano would make more sense geographically, but if you're more interested in coastal scenery than art museums, maybe it would make more sense to eliminate Florence.
Edited to add: A lot of museums and other sights in Rome are closed on Mondays, so that is something you need to be very much concerned about. Guidebooks normally cover this issue, but once you have a list of places you want to enter, you should check for current online info.