That SSD should work just fine in your laptop (formatted correctly, etc.) As an aside, your SATA cable that runs to the Hard drive may be at fault for the slow boot. These are very much known to go bad in the 2012 MBP 13, they do go bad in the 15" as well. They run about $14 in the U.S. and are easy to replace. Considering the hard drive is 10 years old it should be replaced anyhow. An SSD will make a tremendous difference in the speed of the computer. I don't know why it would run hot though.
The steps I would make is:
Remove the Drive.
Replace the SATA cable.
Install the SSD.
Put the hard drive in an external USB 3 case. (an inexpensive one cost about $10, give or take)
Boot the computer via the external hard drive, format the INTERNAL SSD.
Use carbon copy cloner to clone the Hard drive to the SSD. CCC is free for 30 days.
Reboot via the internal SSD.
Most general use is just fine with 8gb. If you photo edit, etc. more intense applications it will also work fine with 16gb ram (despite what Apple says).