Bristleworm is the common name for Polychaete Worms. Polychaete means "Many Bristles". This includes Fireworms, Spaghetti Worms, Hair Worms, Feather Dusters, etc.
Fireworm is the common name for Bristleworms in the Amphinomidae Family. This includes all of the pink/brown/red worms with the 4 rows of tufted white bristles commonly found in live rock and substrate. Almost all of these are beneficial detrivores. There are only a few species that are not reef safe.
The size of the animal has no bearing whatsoever on if it's safe to keep or not. It's the species of the worm that matters. A big detrivore eats the same thing it did when it was smaller, just more of it.