re: I have already imported the users. Will switching to Connections and adding the link extension keep their passwords and other meta data (phone# and streetname) in place?
The Connections data is separate from the user data (by design). Since you say you’ve imported this data, I assume by CSV. You could use CSV Import extension to import that data into Connections. You’ll still need those user accounts so those users will be able to login. You’ll want the Link extension so the user accounts will be linked to their Connections entry.
re: can I have a api callback checkbox with mailchimp?
Connections has no MailChimp integration, that said, Connections also does not do user registration where you would handle this. I suggest a user registration plugin which has MailChimp integration to get those features.
Hope that helps!