Yes, it is free. the only thing not free on skype is if you make calls to landlines or cellphones. all calls between skype users are completely free
yes. skype video calls are probably the most secure form of video calling.
Right click the contact you wish to call and select the second option "video call". Alternatively you can right click and select "call" and click the camera symbol while on call to start your camera.
yes you can
yes as long as you only talk to people you know not strangers it completely secure