Can I just move SSD from 13inch MBP to 15 inch? Mid 2010 13 inch MBP is under 10.11.6 while the 15 inch is a 2012 non-retina model that is under macOS Sierra. I've been told that this is possible earlier but would like to hear form more people. Some are saying Macs are hardware independent, some say they are, so I need a consistent answer. The 15 inch currently has a HDD which is not going to cut it for me. 

What do you suggest I do to move my 13 inch's SSD + data into the 15 inch safely. I have a bunch of environments setup which would take so much time to set up again, so I'm trying to make the switch safely.
Yes you can make that move. Whether or not your environments will transfer I cannot say: it depends whether they are hardware-independent like MacOS. You should have at least one (possibly two, if you're very concerned) backups of your data. These can take the forms of: clone the drive (using Carbon Copy Cloner or similar), time machine backups, or a disk image of the entire SSD. This way if there is a problem during or after the move, everything will be recoverable. After making sure your data is secure, follow a repair guide (I have found that generally iFixit and Other World Computing make good tutorials/video guides) to remove the SSD from the first computer and put it into the second computer.

A few people recommend doing NVRAM and SMC resets before booting the new combination. Try doing these and then booting. Test your apps and environments to make sure everything is working okay. I'd suggest keeping a backup on hand until you know that everything worked. If things didn't work, you can try wiping the SSD and restoring to the new computer from backup, or if you want to revert back to the old setup you knew worked, you could do that as well.

Good luck! Make sure that you understand everything before you do it. Feel free to ask questions or challenge my answer if you have heard otherwise.