It seems that Boost and Virgin phones are, as far as the network is concerned, Sprint phones. Does that mean that it is purely Sprint policy that prevents one from adding a Boost to my Sprint family plan?
Moto G - US Cellular, Virgin Mobile (XT1548)
LTE (2, 4, 5, 12, 17, 25, 26)
CDMA (800, 850, 1900 MHz)
TD-LTE (41 TD2500)
Note that the US Cellular and Virgin Mobile models are NOT unlocked, they are locked to those specific carriers. Plus, Sprint does not allow third-party devices on their network (except Nexus devices), so if you want a Sprint Moto G 2015 you will have to wait until Sprint sells it.
Remember that in Sprint devices, the SIM is only for LTE, which is only for LTE data... For the voice/text and 3G data portions, the phone's ESN/MEID needs to be registered for CDMA service to work, GSM devices do not have a MEID they have an IMEI number. If you get a device that has an MEID and the proper bands, there is still no guarantee that Sprint will activate it even if it is technically compatible, Sprint (and most CDMA carriers) are under no legal obligation to activate a 3rd party device that is not tested and verified compatible with their network (ie. they sell it). Like it or not, services like Sprint, Verizon, US Cellular, and other CDMA carriers have their service locked down. The only exceptions to this rule have been Nexus devices, this year's Moto X 2015 "Pure" edition, and some iPhones.