Small startup: Cassandra + Hadoop or ElasticSearch
If you foresee that moving to Cassandra is definitely in your future, it will be easier to do while your dataset is still small and manageable.  Also, as you learn and get a feel for Cassandra, a small dataset is a better one to make mistakes on (and thus, easier to correct them).  That way your data model is solid by the time your dataset gets big, and that's when it really matters.  And IMHO, there is no such thing as "too small" for Cassandra.

One of our applications uses Cassandra and ElasticSearch in prod.  Based on those experiences, I would offer some caution about using ElasticSearch as a persistent datastore.  We've seen it lose writes fairly often.  Have a read through this discussion on Quora, appropriately titled: Why should I NOT use ElasticSearch as my primary datastore?  That being said, it works great as a search engine.