Yes a 13mm chain can be cropped, however it is going to take some time and pricking about with some sizeable bolt croppers. This will be a significantly better deterrent than some of the things that try to pass for bike locks.
At the end of the day if the crim is tooled up enough they can get anything no matter what you use however if you make it significantly awkward enough for the average crim they will go else where as it is too much hassle.
I have a squire ss50 on my chain and it seems a pretty decent bit of kit.
One word of caution on the chain you plan on using would be to make sure it meets the requirements of your home and contents insurance , some specify solid secure gold chains etc which yours may not be.