Just came back from YYC today on a 6:30am flight. Hopefully this information is useful!
Arrived at rental car return around 4:50am, checked in @ 5:00am (confirmed that you cannot go through preclearance more than 2 hours before flight time). There was about 5 officers without anyone in line at 5:15am, so we got through preclearance smoothly and was done with security at around 5:25am.