I'm pretty sure that the least experienced batsman who plays in Galle will bat at 6, but I hope he bats at 4.
Having a left hander sandwiched between the experienced Ponting and Clarke to me seems like the smart move to make, especially as we all know Hussey is the sort of batsman who can bat with Haddin and the tail. To add to this, Clarke has done massively better at 5 compared with 4 and Marsh and Khawaja have routinely batted in the top 4 for their respective states.
1 Watson 2 Hughes 3 Ponting 4 Khawaja/Marsh 5 Clarke 6 Hussey 7 Haddin 8 Johnson.
R,L,R,L,R,L,R,L