admin bar links remove and add sitewide

hey

i can add and remove links from the admin bar, on a theme by theme basis by adding to the fuction.php.
however i would like to remove the links sitewide, and add some links sidewide.?

also slightly unrelated, in my-sites at the backend, and also my sites in the admin bar, all the sites seem to appear, even if the user is not admin. so say for example if a user is a subscriber for my main site it still appears in my sites. any way of remove this.?

and the lastly, wordpress automatically defaults to contributer when a user is registered. how can i change this is to subscriber (automatically)?

cheers