As of a few years ago, the Skywire were _NOT_ compatible with the Mayfly. They use the XBee footprint, but they’ve re-arranged some of the pins so that it won’t work with the Mayfly – 0.5 or 1.x. At the time, each Skywire variant used slightly different pin-outs so you couldn’t even exchange between them directly.
I don’t know of any other cellular modems that are directly compatible.
If you’re willing to use jumper wires, you can pick any modem break-out you want.