When I do fish, I usually make a sling out of foil and do not have it touching the grate, but I have never noticed an off-taste after doing fish. I typically smoke fish in the Weber kettle.
The thing about fish is that it will be overpowered if you give it the same smoke treatment as ribs. What I would do is have your ribs finish about an hour before you plan to eat, then add a small chunk of alder or other very mild smoke wood, open up the vents and do the salmon by itself.