Using sim cards on another phone is quite common over recent years.  In the past, it was very hard for using a sim card on another phone since they were not compatible due to the difference in network technologies.
There were two sets of networks; CDMA network whereby the phones did not require a sim card while GSM required a sim card. The service providers who use CDMA include Verizon and Sprint mobile. AT&T and T-Mobile use GSM network technologies.
Safelink is a subsidiary of TracFone and Tracfone uses Verizon's and AT&T's network while Boost is a subsidiary of Verizon wireless.
Safelink Sim card will only work on GSM so it is not possible for using a safelink sim card on a Boost mobile phone. But, with the 4G network in place under LTE network technology, you can use a Safelink sim card on a Boost mobile phone.
To achieve this:
The phone should be unlocked Safelink Sim card should work under 4G network technology
So, make sure the sim card is compatible with their network