Commenting or messaging a user to follower her or follow back

No, it is not against the rules but many people find it impolite and 'pushy' if people ask them to follow them. The number of followers you have has absolutely no bearing on earnings or status or anything like that. If somebody wishes to follow you, they will be more likely to do so of their own accord and it, hopefully, means that they will be more likely to see and respond to what you post. Asking someone to follow you is more likely to have the opposite effect! When you follow someone, they should see it in their notifications, so there is no need at all to say so in a comment.

I don't think it's against the rules. I do get PMs from a fellow Mylotter friend from time to time to talk privately. As for following, it's up to you if you want to follow her back.