I would not depend on a taxi for that kind of trip. Many of the taxi drivers do not speak much English, and are taxi drivers, not professional guides. You could get lucky, but it would be a gamble. You are also going to find that the taxis want at least 300 Euros, which is a lot if you are not getting a professional guide. If you hire a decent professional driver/guide you would be able to do Pompeii and the Amalfi Coast..although you might find yourself rushing at Pompeii. Since we do not use drivers/guides (we drive ourselves) I cannot personally recommend a guide. But, there are quite a few highly recommended guides who work the Naples port (you can try google). The downside is that a private guide can cost up to 500 Euros so you do need to shop around.