Sorry for the slow reply, I didn't get an email notification about your post.
Once a user logs in/signup via the plugin, will they be established in normal rainlab.user?
Yes they will. It works like this:
If a user has never signed in with a social account before, the plugin retrieves the email address associated with the social account, and attempts to attach to an existing Rainlab.User with that email address
If a Rainlab.User with that email address doesn't exist, one is created for them.
Twitter doesn't return an email address like most/all other providers, so a new user with a @dev.null account is created for this user.