Can I use the above solar panel to power my arduino just by connecting + to VIN and - to GND?
It will work sometimes. IE only when there is enough light. As long as there is good light it will work.
A 6v panel will produce a higher voltage in bright sun. This will be sufficient to power the Arduino. When the sun is not bright, the voltage fed into the regulator will be insufficient to supply the voltage required for the Arduino.
It may not be the best panel for the job. It may be suitable to charge a battery and use a switchmode boost to boost to 7.5v.