[Jack]

Tater, wet flies and nymphs are actually imitating the same thing- the insect in it's underwater stage, before it rises to the surface and hatches. In appearance, wets tend to have hackles or wings like a dry, nymphs don't (that's a generalization and there are exceptions).
For most stream fishing, you don't need a sinking line to fish a nymph- just leave the floatant off your leader and/or add a bit of weight. Generally in streams, you aren't trying to get that deep (another generalization).
Sinking lines and sinking tips are for trying to fish deep water- say deeper than 10 feet. You will also find that weighted lines are a PIA to cast. Unlike dry fly fishing, you generally have to bring a weighted line all the way in each time, then get the line all the way out again. Frankly (my opinion), any time a weighted line is called for to get really deep, it's time to go to a spinning rod.
When to use nymphs? Any time you're fishing. Remember, those dries you're imitating were nymphs five minutes ago. IME, unless there's a real strong hatch on, there are more nymphs being taken than dry flies, at any given time. JMO.
You can usually use a bit heavier tippet when you're fishing nymphs- and often a shorter leader. The bad news is, it's harder to tell when the fish strikes. Get your Polaroids out, and look for the fish to roll- that roll is the fish taking your nymph and returning to his lie. A strike indicator can help you, too.
The real advantage to nymphs is that you can put the fly right by the fishes nose- not 4 feet above him. That is often too much temptation for a fish- even if he isn't actively feeding.