The PowerBook 1400 would work. It's floppy module (which it looks like you would have to get seperately) definitely supports 800k disks, and should support 400k disks as well. If by AppleTalk you mean LocalTalk over the printer/modem port then no, none of the G4s share any ports with the 1400 line. However, with an Ethernet card popped in you could connect them to the same LAN and run AppleTalk over Ethernet.
eBay should be able to source you a PCMCIA Ethernet card. There were many supported by Mac OS 8 and 9 on the PowerBooks. Maybe someone else can comment in on specific supported model numbers. I don't recall them off the top of my head.
AppleTalk between the PowerBook 1400 and some older Macs like the Plus and SE is very doable once you have them up and running. As for the 128 and 512 varients I'm not sure, I've never used those two machines.