If you are thinking about the western Lake O tribs, I would wait a few additional weeks as there are often not fish until later in september. Those streams are much more dependent on rain but someplace like Oak Orchard should have fish by the 3rd week of september or so (they can come earlier though)
As far as guides, I would think about how you want to fish. There are a lot of bad guides up there. Personally I would find one that specializes in swinging flies. Greg Liu or Jay Peck would be options for the salmon river. For the western end, you could see if Cattaraugus Creek outfitters fishes up there. They have some great guides but are probably more steelhead focused.
Honestly, if you have done much steelhead or salmon fishing you really don't need a guide unless you want access to a drift boat for the salmon river.
I just typed all of that and realized you may be actually asking about fishing in Lake Ontario itself and not the tribs. If so i cant help you but summer is generally the time.