An SE/30 is not "basically a Mac II inside a SE's case"
The Mac II is a 68020 processor, and the SE/30 has a 68030 processor, so the architecture on the board is quite different.
But, the Mac IIci also has little similarity to the Mac II, either.
As to your question - the SE/30 does not support expansion cache.
That doesn't prevent you from trying it, you won't hurt anything - and I don't think the SE/30 would boot with that IIci card in place.