Is it safe to use the USB wall charger I use for my Sansa Fuze to charge my Clip+ and Clip Zip as well? (Using the appropriate USB cables.)
If not, which specs does a charger need for a Clip+ or a Clip Zip?
Thank you! 
Yes.
And economical too, since you don’t need to buy different chargers for different devices.
To add things up, USBs also have a protocol, regarding the voltage it will use. To summarize it, every USB port is compatible with every USB devices. 
Compatible, yes. You’re talking about a USB out from a computer to a USB device. That’s a standard voltage.
But wall chargers and battery needs vary. Some USB wall chargers are weaker than others. For instance, the one that charges my Kindle via microUSB doesn’t have enough juice to charge my phone via its microUSB.
So it’s a reasonable question. Apparently the whole Sansa line charges with the same voltage.
