You can set the redirect URI to localhost or just to your base website address if you’d like. Twitch won’t redirect to that URI except in the case of the OAuth flows. Just an note: you will need to set the URI if you ever need OAuth.
For the Client-ID, you could certainly do it under one Client-ID for “Owyn APIs” if that’s how you’d like to approach it. The Client-ID is simply used to identify your application.