No that won't work because the default pins etc for a Raspberry Pi (which is what the default Rasplex build is designed for) are not the same as the Slice.
Things like the GPIO pin used to control the LAN9514 is different and therefore it is not coming out of reset (which is why you have no USB)
To make this work you need version that has device-tree support (there should be an overlays directory in the fat partition)
Then you need to copy over the slice device-tree files from the standard slice build (slice-overlay.dtb)
Also need to copy over dt-blob.bin
Then need to add
To get slice booting correctly