Can Airport Extreme/Express be used with another wireless router like ASUS RT-N56U?
Yes, if the AirPort router connects to the other router using a permanent, wired Ethernet cable connection.
No, otherwise.
The one exception here would be if you only want to use the AirPort Express for AirPlay. In most cases, it will connect to the third party wireless network.....but....it will not repeat the wireless signal from a third party router.
And will it support multiple hard drives connected to it?
The AirPort Express will not support a USB hard drive at all.
The AirPort Extreme will support multiple USB hard drives if you use a powered USB hub, and the drives are formatted for Mac in Mac OS Extended (Journaled).