Hey Christina, there was a thread a while back about how to make engagement streams happen on more 'randomized' days. The idea of it was that as soon as a cast happens (in your case, maybe when a specific email is sent), there is a smart campaign that pauses someone's engagement stream for x number of days. After that wait step, they are unpaused. The engagement program is set to do casts on multiple days of the week, so that they'll receive the cast shortly after they're unpaused.
Example: Engagement stream is programmed to send casts on Tuesday, Thursday, Friday... Cast 1 for record A is sent on Tuesday, and then a smart campagin pauses the user for 1 week. After 1 week they're unpaused, but at that point, Tuesday's cast (where they would have typically received email 2) has already passed, so they wait until Thursday when the next cast happens. Record B enters the engagement program on a Wednesday so he receives the first cast on Thursday, is paused for a week and unpaused after Thursday's send so he will receive the cast on Friday.
Edward Unthank also did a great presentation at Summit about how to ensure that someone doesn't receive the same piece of content or an email that they've already received using Exclusion program status. You can
view the recording here, and he gets into the exclusion status around slide 17.
Hope this helps a little bit!