Sort of the same issue with some good news: I bought an T-Mobile Ipad mini (2nd Gen/Retina). I decided T-Mobile's coverage is insufficient in my area for the device to be usable, so I want to put it on my Verizon "share everything" plan. According to http://www.pcmag.com/article2/0,2817,2426795,00.asp, it should work.
To confirm, I took the "nano" sim card from my Iphone 5S and put it in the Ipad. WORKED GREAT! So we know it can be done. But, no one at Verizon (call center or local store) knows HOW to activate it. And they wouldn't give me a sim card to try. So I just ordered the sim card from Ebay and will try it out. Maybe they'll let me activate it in the Iphone and move it over. According to a post on MacRumors Forums, by giving the Verizon person "the ICCID number from the Ipad menus," they were able to get it to work. We shall see.
By the way, the Ipad box says, "Supports: CDMA, GSM, UMTS, and LTE celluar technologoies." So, again, ALL NEW IPADS AND IPHONE 5S's (not 5C) will work on ALL major networks, period. (Those of you who have older Apple devices, quit telling people they aren't unlocked...they ARE!)
But clearly, this doesn't mean the reps at your particular carrier know HOW to activate it. Duh.