@mgglim: the sensor is assumed to be dry contact and does not require power, hence there is no issue with it being AC or DC powered. Most rain sensors are of this type, and a lot of flow sensors too. Internally, these are simply mechanical switches that are activated by rain or water flow. If your flow sensor requires power, it often can still be used with the sensor port, as long as it sends digital ‘pulses’ and not analog signals.
The OpenSprinkler firmware allows you to flag each station individually whether it’s affected by the sensor or not. So if you want the sensor to only affect one zone, you can turn the ‘sensor’ flag of the other zones off. This flag is in the station attributes (which you can access by clicking on the gear icon next to each station at the homepage).