Hooks, Notification Settings and more

Hey,

I just switched to IPB 4.1.8 and got some questions.

1. I want an clean install of the files and stuff, but I want to keep users, forums, posts and stuff. Should I upgrade from 3.4.7 or is there another way to import all those data?
2. I want to force the option "Show popup window when I receive a new personal message" to be activated by default and don't want the user to change it. Is there a way for it? I just can find the other notification options in admin notification settings.
3. I want to make a plugin or a hook or something to change the color of the username. What would be the best way? I think in 3.4.7 I had a hook for it. Is there any documentation about the plugins, how to make them and stuff?

Kind regards