Due to changes between ES 5.6 and ES 6.6, starting ES 6.6 within SQ is a bit different and require some changes.
ES 6.6 distribution has grown and embedding it will impact the size of SQ packages. Let's try and reduce this impact as much as possible.
ES 6.X introduced changes at Java API level. Those changes should be easy to get around thanks to compilation and ES documentation.
Other changes apply to search, queries, ... may impact SQ and will be harder to confirm and fix if need be unless testing (UT/IT) would catch them.