An AT&T nano SIM card (from an iPhone 5 or 6, for example) will fit in the Electron and should work, at least without the Microcell. To fully connect to the cloud you’ll need to insert this line in your code:
STARTUP(cellular_credentials_set("Broadband", "", "", NULL));
Unlike Wi-Fi settings, the cellular APN is not stored in configuration flash so you always need to add that line to any program that uses a 3rd party SIM card. The string “Broadband” is specific to AT&T. Just take the line out when you put the Particle SIM back in.
It’s certainly worth a try if you have all of the pieces on hand.