I support what everyone as been saying concern the uses of 6inchs block to construct a building without mix it with 9inchs blocks, sure the 6inchs is enough since it not the bricks that carried load, it's the column and the beams that carried the load and secondly also depends on the results you got after the soil test to determine the kind of foundation to be use and structural drawings will also be helpful as,

But mind you, weather you use 6inchs blocks or 9inchs block you will still get the same space, because everything will design and started on the architectural drawing.

Thank you