1) Both the sensors you mentioned should work with OSPi. You can simulate a flow sensor by connecting a switch (or simply two wires) to the sensor port, and clicking the switch a few times (or equivalently shorting the two wires a few times) to simulate the clicking of flow sensor, and the real-time flow rate shown at the bottom of the homepage should change (it updates every 30 seconds I believe).
2) As I said, the refresh rate is 30 seconds I believe. This is because it needs to accumulate at least several clicks before it can measure the flow rate accurately. This is especially the case for flow sensors that have low pulse rate (i.e. smaller number of clicks per gallon). You can increase or decrease the refresh time by changing the parameter in defines.h and re-compile the firmware.
3) The microcontroller-based OpenSprinkler (either AC or DC) has current-sensing circuit already. OSPi doesn’t have current-sensing yet — but it’s possible in future revision since there is a ADC chip on board.
4) The data is very small because the weather script is hosted on our cloud server, so when it queries for weather, it merely sends a request to the cloud server and the script on the cloud server does the computation and return the result to OSPi. By default the firmware queries the server once per hour, but you can make it less frequent by increasing the time interval.