My thought was to let you have e.g. a book review bundler that got only the review posts from mixed sources and say 1/3 of the review posts from ones that are a

My thought was to let you have e.g. a book review bundler that got only the review posts from mixed sources and say 1/3 of the review posts from ones that are all reviews ... both because that sort of subsetting works better here than it would for apps and because it means that it can make sense to subscribe to both a bundle and a contributor thereto, so neither channel is risking cannibalisation by recommending the other.