Thanks for the quick response!
Interesting, why can I still set a retention rate when it's not needed? 
Anyway, driving the question a bit further to hopefully tackle down my problem.
Lately I run into an issue, that my follower index never replicates anything. It just creates the index and does not fetch any data from the leader. It doesn't matter if I reduce the load. Thats why i was thinking maybe a premature merge could have caused the problem. When I call the ccr-stats API for that index, it tells me, that it's not able to fetch the licence information. I explained that in more detail over here: CCR - Remove follow_stats and License fetching problem