The newest version of NightBot has the option to use a playlist created by you (or anyone else for that matter). You can then limit song requests to songs in that playlist. It is also possible to import a playlist with the latest beta version. If you can find or create a royalty free playlist and import it into NightBot, then you’ll be all set. Otherwise, doing song requests wouldn’t really work for your usage needs. I just did a quick search on youtube and found this: https://www.youtube.com/results?search_query=royalty+free+playlist
It might proove useful to you.