Is swallowing pieces of plastic from night guard for grinding teeth dangerous?

No. Anything tiny and non-toxic that is small enough to be ground off a guard by teeth activity, if undigestable, will pass harmless through your GI tract and be eliminated in your stool. Don't give it a second thought.

Not really. As long as the pieces are small.

Probably not. If it was a small piece of plastic, odds are that it will simply pass right through you.