When I buy a package of streams, do I have to declare the site or app where they will be used? Are Streams “locked” to one site or app?

No - you do not need to declare where the streams will be used. The streams are yours to use within any domain or app. We do require you to specify domains (for the JS or AS apis) - this functionality is for security purposes, to protect your account from Stream theft, and it is not license driven. You may add as many domains as you need.