I doubt Cyrus the Great was ever addressed as “Cy,” so you probably don’t mean him. Likewise, I’d guess Cyrus Vance, Jr., the DA of NY County, is probably not “Cy” to all but his closest friends. As a matter of fact, no matter who Cy is, if you don’t know him, maybe he’s not “Cy” to you. Call him Cyrus to begin with, and if he invites you to call him Cy, then you can. Whoever he is.