The 1100 is ARM and there are no CE ARM builds of any version.
Even the older versions before the "Plus" branding were still customized and referred to as "Factory" versions which were different from CE.
Some of the amd64 Netgate hardware can run CE, but some require special drivers/code for certain hardware and you'd lose other hardware-specific tuning and capabilities. For example the 7100 wouldn't run CE well because it couldn't drive the integrated switch, among other things. The 6100 could run CE but you'd lose QAT support and other items tuned for that hardware (LEDs, Reset button, and so on)