Possible? TC to backup MBP & mini & USB-HDDs

Hi Experts,

maybe its a newbie question, but I didn´t found anything that helps searching the forums....

Thinking to buy a TC for backup purposes, but I´m not sure if its the right decision for my environment:

I´m using

- Airport Extreme as base connection to the internet
- two Airpot Express to extend the range
- MacMini
- external 300GB USB-HDD connected to the mini for the iTunes library
-external 500GB USB-HDD connected to the mini for Time machine
- MBP

Up to now I didn´t manage to

- connect the 500HDD to the Extreme to use it with TM for both mini and MBP, so I´m switching every time I want to have a backup of the MBP
- connect the 300HDD to the Extreme to access my music from both, mini and MBP
- configure iPhoto to use the 300HDD

... but thats not my question now

My question is: would it make sense to buy a TC to connect it to the Extreme for backups ?

In general I suppose thats not a problem, but...:

- would it be possible to make backups wireless on the TC from both mini and MBP using TM on both (via the both Express´s)?
- would it be possible to make backups of the USB-HDDs (connected to the mini) as well ?

Any suggestion where I can find some configuration hints is appreciated!

Best scenario I can imagine is

- using the Extreme for internet access
- connect both the 300HDD and the 500HDD to the Extreme
- using TC for backing up mini, MBP, 300HDD and 500HDD
- configuring Mini and MBP to access music and photos from the 300HDD/500HDD

Any hint is highly welcome!

Regards,
Michael