Can I use DC20V/6.75A instead DC20V/6.5A Power Adapter
If it both stabilized output power supply will be no problem.  Not stabilized power supplies rated with output voltage with current, close to maximum, at lower load output voltage will be higher and may cause damage. Also the primary power supply may need overcurrent protection. Best to use original PS, using another should take more thing in consideration.  More info needed to  answering.