I am not sure if there are any fly fishing guides on the Niagara. As i am sure you are aware, it is a huge river and not easy to fish. I personally don't love it but would head down there to kill a couple hours every now and then. I don't know if i would consider it a whole day kind of place. I would say november is probably the best time, especially if you have any interest in catching lakers. Steelhead are there all winter but it can be pretty miserable in the gorge at times.
Be extremely careful wading. Like stay within a foot of the bank.
Oak Orchard Flyshop can provide more info.