I noticed that I can make a rule/notification to let me know when/if my fridge/freezer door is left open. How can I set this up? Is it with a typical IQ door/window sensor and learn it in as a non-reporting.
Yep, you would just use any surface door sensor and set it up for group 25. You can then set up notifications in Alarm.com.
Also. if I want to have a contact on my shed door, which is not winterized, I noticed the IQ Slim W/S contact is rated for -10 to +50 oC, Definitely OK for the hot end, but winters can see -20oC, for a couple of months. What exactly happens when it goes that low?
Both temperature and humidity are a problem for sensors and any normal indoor sensor should never be used in an outdoor environment. While it will cost more, there is no reason not to go with a sensor that is rated for outdoor use.
Look at the PG9312.
An indoor sensor will very likely fail to corrosion due to humidity and condensation, and before that temperatures outside of the supported range may affect transmission range, battery life, sensor sensitivity etc.