I have a 9yo mutt who probably has some setter and/or runner breed genes. Her face looks a lot like that collie+setter from the last guess the breed, but she’s short-haired and more aerodinamic/skinnier. While she’s very obedient when off-leash, I have trouble bc she wants to say ‘hi’ to every dog we meet at the beach AND she’s an explosive runner. No dangerous incidents so far bc she is super friendly but knows when she isn’t liked, but some small breed onwers frown at me sometimes. Any tips?

I would only allow her off leash where it is acceptable. If it isn’t a dog beach, then she should be on a leash. If it is a dog beach, and she is being friendly, then she is fine. 

You could try a training lead or retractable leash for the beach so she has pseudo-freedom, but incorporate training by not allowing her to practice her behaviors and fixing a command to the act of leaving others alone. Everytime she wants to romp over to a friend, give a command and reel her back to you with either a treat waiting or if she enjoys anything else like fetch, running with you, some type of reward that is better than greeting a stranger. 

But again, if it isn’t a dog beach, it might be best to keep her leashed.