ZGC relies on the space of 64-bit pointer (and the size of the 64-bit virtual address space) for its pointer coloring algorithm, and so it is by definition incompatible with UseCompressedOops even at small sizes.

I suppose it's possible to fix this with a change to the design in some way. But if you have a heap smaller than 32GB, the existing G1 would probably work well too. ZGC shines much more on very large heap sizes since that's where long tail GC latency often hits hardest.