Safest way to activate a "backup" iPhone, to be kept on a lower version of iOS
Don't worry , you can use the SIM card to activate the iPhone SE.

You can always make backup copies, regardless of the iOS version you have. An important fact is that in order to restore an old version, the version you are going to restore must still be signed by Apple. Normally they leave the 2 most recent versions available, in these moments iOS 12.2 and iOS 12.1.5.

You do not have to worry, they will not remove the Jailbreak or anything like that by activating another iPhone.