I just reserved a man who has a LOT of good reviews on this board, Renato Cuomo. His website is www.cuomonet.it and although he is based in Sorrento south of Pompeii, his price was better than Rome companies. He also has a tour guide for Pompeii he recommends. Having been to Pompeii, I highly recommend a guide.

Lot of people talk about taking the train from
FCO to the Termini and then hopping on the metro or the bus.

But if you have any amount of luggage, this may be impossible as the metro trains and busses are often full, standing room only and packed at that.

A taxi maybe.

There are some affordable shuttle options like on viator.com. $17, not euro, each way.