If it's your first time to Italy, I don't think Cinque Terre is worth the detour, nor do I think Genova is a great place to visit on this trip. Rome is a very obvious choice and I think you should stick with it - it's very close to Florence, which will maximize your time actually doing things, and 4-5 days will go by in a flash in Rome. Rome and Florence are two of the three most popular cities for first time visitors for a very good reason - they're worth seeing.