The only smaller cards I have are old.
256MB Ridata card ~6-7yrs old
10MB and 4MB Kodak cards from ~1999
Some of my experiments with CF cards in machines are documented here: http://synack.net/~bbraun/idecf.html
The Kingston and SanDisk cards work in the same adapter in many other machines (currently have Kingston cards in an SE, IIx, Beige G3, and Pismo). From what I've read, the 630 has a bit of a quirky ATA controller which would explain some pickiness on the cards.
Rob