Let's just say that the rates you will get around the western oriented hotels will not match the rates my SL friend will get me from his sources around Gampaha (not that far from the airport) I have been pleasantly surprised that most drivers I have encountered speak some English though few with the vocabulary to also be a tour guide. But many with just basic English are very quiet. I wouldn't be shy about asking the (many) taxi desks on the way out of immigration if they would consider "touring packages" though I would expect no bargains. You might do better in Kandy.

And there should be two rates - a mileage rate for local trips (one day) and a daily rate for touring.

Fred