I cut down a Telus SIM and had 50% success. Basically, the iPhone read the SIM and allowed me to finish the iPhone activation processes and use my device. What won't work however is the cellular. The phone registers no service so I'm limited to Wi-Fi for now.
Perhaps I was not accurate enough when I cut around the SIM template I found online but for now, cutting didn't pan out 100% for me.
About Chatr specifically, I'm not sure if the iPhone 5 will work on their network. I would give them a call to confirm. If you bringing an unlocked device to their system, it should in theory work. If you cut your SIM correctly that is. )
Let us know how you made out please.