I was wondering if it is safe/plausable/possible to plug a usb hub into the USB port of the Airport Express to connect my two hard drives (and maybe a printer) to connect all three of these devices to my local network?
Unfortunately, the AirPort Express will not support a hard drive connected to its USB port. Only compatible printers are supported.
So, while you could connect a powered USB hub to the AirPort Express, it would only function correctly with multiple printers.
To support a hard drive, you would need an AirPort Extreme.