phario, welcome in advance to MT. October is a great time to fish here. There are really no crowds and you have the rivers, for the most part, to yourself. If this is just a fishing trip, I assume you are spending a lot of money for the experience. You could fish every day for 6 months straight and never experience all that Montana has to offer.
First, spending several days in Yellowstone National Park is good advice. Realistically you could spend your whole two weeks there and probably not be disappointed.
I would recommend splitting your trip into three separate blocks of 4 to 5 days each. I would then suggest to pick three rivers. My choice would be 3 of the following: Missouri, Madison, Big Horn and Yellowstone (throw the Yellowstone Park Rivers in as a 5th choice). On the first day of each of those 3 different blocks, I would hire a guide. It would cost you anywhere from $400 to $500 (I think 200 to 300 pounds) per day. After enjoying the benefit of the guide on your fist day on any given river, spend a couple of more days yourself on the same river. Montana has the single best stream/river access laws in the world.
I don't think hiring a guide is a necessity; but, if you are spending the money to travel here strictly for fishing, I would hate for you to miss the best we have to offer. Hiring a guide would/should accomplish that for you.
If you would be willing to pay for the post, I would be happy to mail to you several good books on fishing in MT to help you make up your mind. All I ask is that when you come to MT that you bring them with you to return them. If you wish to take me up on my offer, send me an e-mail at firstname.lastname@example.org
to make such arrangements.