Foundation is a very important stage and need to be get right at once, since there’s no soil test and only you can predict the soil nature I assume u use 9inches hollow block since it Will bear the load coming from roof..
The 9inches should cover important walls in your build (External walls and some internal) the 6 inches can likewise be use for some internal partition wall according to the architectural specification.
Mix Weak concrete and broken blocks to fill up this 6inches block.
If the area is larger kindly make sure you backfill the area.
Kindly mention back to me for clarification typing this while walking