Im trying to put together a (cheap) dual stage water/meth inj setup that will be triggered by megasquirt, it has configurable outputs and ive made a diagram of my intentions below.
I've also found a water flow switch that gives a 0-5v output linear to flow but would like to have a dash gauge that will let me know when water is flowing through the first nozzle and then the second.
This can also be wired back to the megasquirt as a failsafe and switch ignition maps if solenoid fails or tank runs dry..

Is there a universal 0-5v gauge that i can use?
have a look see if you think this will work... trying to avoid the expensive kits.

You mean a flow switch? Depends on the minimum flow rate to trigger switch. What flows are you talking about and what bores?

could you use a fuel flow sensor?

http://www.race-technology.com/content.php?pcat=2&cat=728

should flow approx 2gph at 100psi through 6mm od nylon hose.

that fuel flow sensor could do the trick but it designed for data logging and doesn't have and dash visible gauge, sort of the same as the one i was looking at.

http://www.aquamist-direct.com/806-428tm-turbine-flow-sensor-standalone/?setCurrencyId=1

One thing I can suggest is that you try to locate the nozzles above the level of the tank as an extra measure to prevent self-syphoning when the car is now parked for long periods.