Shamim's Plugins

As this plugin use separate plugin and query system, it should not impacted by other plugins.
Also it totally depends on hosting. If you have good server it can go well above that counts.

Yes, possible.
You can follow https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/group-messaging/

It is possible but needs php knowledge.
You can get an idea from https://www.shamimsplugins.com/support/topic/how-to-search-messages-by-username/#post-41820

For this type of code you can contact me using https://www.shamimsplugins.com/hire/.

You can add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_filter_user_name', function( $name, $id ){
	if ( $name && $id == get_current_user_id() ) {
		$name .= " (You)";
	}
	return $name;
}, 10, 2);

Current version it is not possible without modifying core plugin code.
In future i will try to add this.

Please add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_current_user_can', function( $can, $cap, $id ){
	if( 'send_reply' !== $cap ) {
		return $can;
	}
	if( $can && $id && fep_is_group_message( $id ) ) {
		return false;
	}
	return $can;
}, 10, 3);