Bristle worms are actually good for your tank... though they can grow to some rediculous sizes (biggest one we've had was almost a foot long. But they eat detritus and junk, and help stir the top layers of your sand bed. They do have some natural predators.

QT'ing coral is a good practice, in theory the tank conditions should match your display as closely as possible. Though the costs can make it prohibitive to match things perfectly... high wattage PC lamps or T5's may be a good substitute for halides during QT. But salinity, temp, and pH should be as close as possible.

Particularly with the number of pests that are being circulated through the hobbyist channels lately. SPS (red bugs, and flatworms) escpecially, though not many softies will carry unwanted pests (though possible), like the zoanthid spiders that were making the rounds last summer.

All sounds good in theory, but certain 'dips' can be a good preventive measure. Iodine dips for zoa's, Interceptor dips for SPS red bugs....etc, other more specific dips for other corals can help.

BTW what was the hitchhiker fish?