Risk of overloading a usb charger
Damage is unlikely.  Most likely what will happen if you overload the USB charger is that it will either drop its voltage or cut out altogether.  If it drops its voltage, then things won't charge right.  If it cuts out, then it will be like it is off.  It may cycle on for a short time every few seconds while the overload persists.

Actual overload is also unlikely,however.  USB 2.0 only provides for 100 mA per device without enumeration, and 500 mA after requesting and being granted the extra current.  Without special communication, your devices have to assume they are plugged into a normal USB port and not draw more than the maximum current for such a port.  There is a reasonable chance that your devices are well enough behaved to not overload a ordinary USB port and only draw extra power form their special branded chargers after some private communication.  However, this will mean that these devices will take longer to charge their batteries as fast.  You can't have it both ways.