I just returned this week from a seminar taught by Burt Foster on stainless san-mai. It is not an easy task, and requires some advanced equipment. Either the 300 or 400 series can be used. It is not a simple open forge ,flux-weld type operation, and the results are definitely hit or miss. Kevin Cashen had some amazing micrographs of the steel in cross section.
I would make some san-mai with mild steel, or wrought iron sides and a 1095 core first. And when you have it pat, contact Burt ( or go to Ashokan next year) and get the lowdown about stainless san-mai.
Stacy