Wow! nik9000, thanks for the response. I was just reading about your work on the reindexing abilities in 2.3.
So I guess we'll take the Faking Index per User with Aliases route for small users and keep them in a shared system with overallocated shards, and need to figure out when to split out One Big User into their own index.
From this article it sounds like for my "fake index per user with aliases" shared index if I have 4 nodes, then I should initially have 4 shards? Makes sense, but seems too simple. This article on overallocation suggests if I'm reading it right that if I have 4 nodes I might want to create my shared user index with 6 shards. So when I add more nodes, because I overallocated, there is no reindex required and new nodes, will simply start being used.
I think I'm looking for too simple of an answer probably for my "One Big User" scenario and need to spend more time in Designing for Scale docs.
However, if someone has a good rule of thumb in one paragraph or less for determining scale for the "One Big User" or my shared index I'd be interested to hear thoughts.