Post-Head gasket replacement issues?

[CrossTrek]

That thick brown sludge could be "dexkill" if they mixed different coolants, dexcool with another one... Im assuming that your aveo use dexcool...
At the other hand could be oil mixed with the coolant if you have a blown head gasket...
Look at your oil with the oil stick or in the valve cover.
You must demand explanations from your mechanic

Good luck!