1. Simply having a child theme activated won’t trigger that message. However, that message will trigger if the folder name or name of the parent theme has been changed.
You can always remove that message with this PHP:
remove_action( 'admin_notices', 'generate_premium_theme_information' );
2. Definitely don’t add that much CSS to Simple CSS.
You can create a separate file for your contact page, then add it to that page only:
add_action( 'wp_enqueue_scripts', 'tu_add_child_scripts' );
function tu_add_child_scripts() {
if ( is_page( 'contact' ) ) {
wp_enqueue_style( 'my-contact', 'URL TO YOUR CONTACT CSS FILE' );
}
}