Goldfish acting strangely?

That's fine. don't worry about the little one. They could probably have their own language and think that the little guy is cool. They probably wanted a new friend after awhile. How would you feel if you were in the same tank day after day with the same other fish and the same food every day. So this can probably be why they welcomed the new little one along with swimming against the filter to relieve their boredom. ;) This behavior may not be normal, but it isn't abnormal or below normal. But it is still not normal. I hope this helps:)