An internal drive is not a backup drive, unless your primary drive is external.
If the computer fails, due to power surge, dust clogging the fans, or water damage, how can an internal drive be a backup drive?
Yes, you can install a SATA 8 TB hard drive into a Mac Pro mid-2010. But something else will need to be backing it up in event the computer fails and everything on it, and every drive in the computer fails.
Yes formatting it HFS + Extended will work on a platter drive. That will allow it to be used as a boot drive if you installed an OS on it.
High Sierra will make SSD drives APFS automatically.