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As far as I know, a pack could have as many blueprints as it makes sense to have.
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You should have a sample map demonstrating the use of your new blueprints anyway, so you can use the level blueprint of that map and make additional comments in the class blueprints, so the users know they need to look over level BP also. Though I would recommend to make your blueprints self-contained as much as possible, especially for these BPs that are gonna be shared with others.
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You could use them since you’re only sharing with other subscribers, though I would only try to use the assets in the starter content since everyone has them already. As an example, you better use the little blue guy instead of one of the characters that comes with Mixamo pack.
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This one you should wait for an official response, but I don’t think that’s gonna be an issue since the pure focus of your pack is on blueprints and not those assets.