Disc brake mount/adapter bolt length
These bolts are usually 18-20mm long M6 bolts. The exact length isn’t important, but going shorter than 15 will likely mean insufficient thread engagement, while longer than 24 or so could potentially result in the bolt interfering with the rotor.
The plastic snap rings aren’t strictly necessary. They act as a second line of defense against the bolts inadvertently loosening. Shimano includes many of these secondary safety systems with their products (eg. safety clips on brake pad retaining bolts, threadlocker, fancy spring washers under rotor bolts, and these plastic things) which certainly will help, but the bike won’t explode if you don’t use them.