Hi Jason,
Yes, you need to have same version on both switches for creating virtual-chassis(VC).
Virtual-chassis Fabric(VCF) is incorrect terminology in this case as VCF is spine and leaf topology and cannot be formed with just two devices.
Install package is copied to the device using SCP or by mounting USB and installed by command request system software add whereas install media is a bootable image to be loaded into USB and can be used as an emergency boot device. In your case, you should be able to use install package. for more info refer https://www.juniper.net/documentation/en_US/junos/topics/concept/understanding-junos-freebsd-package-naming.html
>Prefix—This is junos-install. This prefix takes the place of the prefix jinstall and the bundle jbundle. We still use the term bundle in the new package-naming convention. Starting in Junos OS Release 15.1F3, PTX5000 and MX240, MX480, MX960, MX2010, and MX2020 routers run Junos OS with upgraded FreeBSD as a guest virtual machine (VM) on a Linux VM host. The installation package for these routers has the junos-vmhost-install prefix.
>Media keyword—Added to the prefix, a media keyword is only used when the image is not for use with the request system software add command. Values for the media keyword include usb for images installed from a USB drive or net for images installed over a network; for example, the entire prefix of your package might be junos-install-usb-.
I would recommend you to build virtual-chassis by upgrading them to same version instead of using as standalone. VC has many advantages over standalone device.
Regards,
Raj Maradia