800mA per zone is reaching the limit of the triac. You should use a relay to drive your LED if it consumes that much current. Also, if you drive LED with AC power, keep in mind you will get flickring (roughly 30Hz). This may or may not be an issue depending on your application. 6A supply is fine — the current rating just means the power supply is capable at outputting 6A while still maintaining 24V AC voltage.