For normal shots second curtain sync will work fine. I normally shoot fireworks at around f/16, iso 100 and 15 seconds or so. You could of course go with a wide aperture and get something that most others don't..........
The Bokeh Show
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As far as silhouettes go, it's a crap shoot, unless you want to put a flash behind them with about a 1/2CTO gel on it. To get enough light to create a strong silhouette you need a relatively long exposure, which will often preclude you from getting a crisp shape. Upping the iso runs the risk of blowing out the highlights, or not getting a long enough light trail to really showcase their shape.