You do not need any approval for posting youtube videos on your hubs. Video having objectionable content (adult content, anti-religious etc.) are not allowed as per google's ad policy. Most of the web master will not object if you link there sites with your hubs but you can not link a particular site more than two times on a single hub. Some sites do not allow to link there content that visitors need to visit personally. For example, you can not link "register or sign up link of a site' if it is forbidden for direct linking. You should read copyright terms & conditions of a site before linking their content.