I just got my PR through Common-law Sponsorship Aug 2016 and let me tell you, I sent absolutely everything I could, along with about 150 photos dating back from 2012 when I met my partner. (My full application was about an inch thick)
You really need 12 months of evidence to prove you live together, like rental agreements, joint utility bills, joint bank accounts, etc. If your relationship is longer than 12 months like you have been together for 2 or 3 years, then show things that prove you have been a couple for more than 12 months like tickets to see concerts, shows or games, tickets to show you have visited places or been on vacation, plane tickets etc. I sent screenshots from all our time-stamped Skype conversations for the time I went back to the UK. I, personally sent about 4 letters from friends/family, and none of them were notarized, but that's simply because getting letters notarized was expensive and I wasn't working at the time
Hope this helps you!