False. Pre-OSX Macs, including 68k Macs can indeed share files directly with Tiger (and probably Leopard too, but I've not tested that yet).
Specifically, my SE/30 is a 68k Mac. I can fileshare directly from my SE/30 running System 7.5.5 with my OS 10.4 G4 Cube. But of course, you have to know the procedure. I've outlined that specific procedure in this detailed thread. Pull up that thread and search for "TCPquantum" in my (JDW) reply to "Doug." That post will get you started. When you have finished reading that page of posts, click "Newer >" at the bottom right of the page and read the posts on the next page. I list a lot of information on both pages, but the second page is a summary of all you must do.
As to getting a 512k to directly network with OS X, I've not been able to do that. But honestly, I've not spent time worrying about it either because I just copy files to floppy on the 512, plop the floppy into my SE/30 and then I can send the files over Ethernet to my Cube.