I have used foreign (Not Palm) USB chargers on my Palm Pre Plus several times without a problem. The only issues I can think of is that the Palm chargers output a full 1 Amp while some others only about 500 mA. This will not power a touchstone but it will charge your phone – it will just take longer. Following is the USB Specification for power:
Dedicated charger mode:
A simple USB charger should short the 2 data lines together. The device will then not attempt to transmit or receive data, but can draw up to 1.8A, if the supply can provide it.
Supplied voltage by a host or a powered hub ports is between 4.75 V and 5.25 V. Maximum voltage drop for bus-powered hubs is 0.35 V from its host or hub to the hubs output port. All hubs and functions must be able to send configuration data at 4.4 V, but only low-power functions need to be working at this voltage. Normal operational voltage for functions is minimum 4.75 V.