Anything can happen. I have one Sepre Tetra and he was accepted into the family of Tiger Barbs. Yes, they do swim together, stay in the group together, and play together.
Chasing them as in defending its territory (most likely) or chasing them as in acctually trying to school with them. I'd think it was the first choice, about territory.
when you have a group of schooling fish and you have a couple fish that school but dont have enough fish for their own school, they usualy pair up with the other group. in my brothers tank he has 12 neon tetras and they schooled with my guppy fry (4 months), they usualy pair up with fish their own size.
I also think he could be defending his territory. The female GBR I had would chase away every fish in the tank...even those bigger than her. She never did any damage, but she was certainly fiesty.
GBR's will sometimes chase other fish if in their territory, but I haven't had that problem in my 75G. They've accepted all the other fish in there, and I have no problems. If he's a single GBR, might want to get him a friend.