As has been stated, trolling speeds for live versus artificial are two different numbers. 0-1 for live, 2+ for artificial. Yes, zero can sometimes be the PERFECT speed if a school is right underneath you.
HOW you are trolling artificial matters as well. If you are just trolling weight, you will want to stay on the low end closer to 2 as the faster you go, the more shallow your baits will run. If you are trolling cranks/spoons, you will want to make sure to troll at a speed that works best for the baits. If you are trolling down riggers, you have much more wiggle room on depth.

I have started doing more of what APB has said when using riggers in trolling faster (3+) to find the zone. When I see marks, I will slow down/stop, allow the baits to fall, then try to "gun" the baits to get them to rise quickly. NOTE: You WILL get strikes on the fall when slowing down, but far less hook ups. Vertical changes are a must. If in a consistent "zone" I will hold steady at 2.5 or so and move in an "S" pattern. This causes one side to raise while the other side falls.

I will sometimes mix baits, but only with one type of bait and bucktails. Never, say, spoons and cranks. Bucktails work at any speed and you can vary them with the trailer. Bucks work with stacking under shallow running cranks, Bucks work with stacking over heavier spoons. Mixing cranks and spoons? Not a good plan unless you really really understand speed and bait variances.