Would like to increase my internal memory that way..How 'safe' is it?
It should be there on the notification after you've inserted that memory card.
From what I am aware of (by surfing about this), it's not available in Android 7.0 Nougat but I'll remove my SD card & insert it again
Have you checked under Settings -> Storage?
Yes, I've checked that.
AFAIK Adoptable storage option is disabled/hidden in Stock ROMs, as the manufacturers provide more inbuilt storage so as to increase the mobile's performance (read & write) - instead of depending on SD card which comes with a dip.