is it possible to create a forum where people can either do a social login
or a login with accounts from other smf?
I can not tell you smf but from common sense I can say if two forum located in two different server then I do not think it will really help. I guess for login authentication you need to communicate with browser's session variable. Same session variable is not gonna work for different server IMO.
It is completely possible to add social login feature for any forum even with smf. If smf currently do not have such feature then all you need to do is hire a developer who can do the job.