Motorola Droid X :: Safe To Charge Using Mini Usb Charge Source?

This has been an issue that's been bugging me the last few days now. The stock wall charger that came with my Droid X is the 5PN5504A, which has an output rating of 5.1V, 850mA. My understanding is that the mini-USB interface is an industry standard for power delivery to portable devices, which I take to mean that any charge source should work (wall charger, desktop PC, laptop) as long it has uses an USB interface. So any USB-->miniUSB cable can be used to charge any mini-usb device, regardless of power source, right?

I already have several other mini-usb devices, such as bluetooth headsets, which all came with their own wall chargers. However, I checked these chargers, and they all have different outputs: 5V @ 200mA, 5V @ 150mA, 5V @ 550mA. Because the default Droid X cable was so short, I was tempted to use these other chargers. I plugged in the 5V@150mA charger to my Droid X, and found that the phone does not show it's charging (the lighting bolt isn't there, there's no animation on the battery indicator). So then I switched to the 5V@550mA charger, and it DOES charge....................