star tree index is not enabled by default right? Any reason for that?

Most indexes need to be configured, and startree is no exception. Depending on your use case you may need certain indexes and not others.

How do multiple indexes work with each other? Like InvertedIndex and startree Index? If we are ever concerned with only aggregates like SUM,COUNT from the table, does having only star tree index makes sense then?

These indexes can coexist. The planner will hookup the optimal for the query. Start tree index helps with low selectivity aggregation queries, if that’s your case