How to customize feed?

Feed templates are special case. You can’t simply provide your own template – you need to work with do_feed_ hooks to change handler function.

But for your specific question it is much easier to override author. Try this (not tested):

add_filter( 'the_author', 'feed_author' );

function feed_author($name) {

    if( is_feed() )
        return 'Acme, Inc.';

    return $name;
}