Are you charging normally, or using the Fast Charging feature? Fast charging causes more heat. If it's getting hot while charging normally, it's possible the charging cable, charging port, or battery are faulty. The only thing you can try is a different cable--if that doesn't help, then you may have to contact Samsung for warranty repair.
There are a number of different cloud storage services available. What exactly are you trying to back up? As long as your contacts and calendar events are associate with Google (and not the Phone/Device Account or Samsung Account), they will sync automatically with your Google account. 3rd party apps may or may not backup data to your Google account--the problem is there is no easy way to tell. So if it's very important for you to backup 3rd party app data, then use
Helium or
Smart Switch. (I used to suggest Samsung Kies, but from what I understand, it doesn't work with the S6 line.)
Photos, videos, and other discrete files can be backed up to the cloud storage of your choice. Google Drive is the easiest, but you have to make sure you have the app installed on the phone. Photos and videos can also be uploaded to your Google+ Photos (formerly known as Picasa). There's always Flickr, Dropbox, Onedrive, Asus Webstorage, etc. etc., so take your pick.