Hmmmm. That's a great question. The easy answer is to just brew a lot, and then you'll know. But I think that's too simplistic.
As a rule, a good place to start when pairing hops is to consider other recipes. Even if you're not copying a recipe, looking at them to see what other brewers do is a good idea. What I mean is this- if "everybody" is using C-hops (cascade, chinook, centennial, etc) for American pale ales and IPAs, that's a good indication that it's a good combo.
Another thing to consider is the heritage of the hops. For example, challenger hops, fuggles, East Kent goldings, etc are UK hop varieties. So if I was thinking about an English pale ale, I'd probably use a British hop variety or two. The same is true with a German lager- German hops would fit well, but cascade hops from the US would be odd and out of place.
As far as plugs, pellets, whole cones, etc, it really doesn't matter a bit. I use whatever is available at the best price. The differences are nominal.