Can a pre-built voltage meter be used as part of the salinometer, as long as it was not previously part of a commercial salinometr/hydrometer?

Yes, a voltmeter could be used as the required hydrometer. There is nothing in the event description that would preclude such an approach (use of electronic instrumentation). The competitor should be prepared to explain where the various parts come from and demonstrate that the student's device is not from a commercial device for measuring water quality.

For the salinometer, are we allowed to make an electronic salinometer?

Yes, STUDENTS are allowed to use electronics in their STUDENT built salinometer, see posted FAQs.

For the salinometer, are we allowed to use a commercial volt / ohm meter NOT used for measuring salinity, then reconfiguring it to do so? Would this be considered a commercial device?

Yes you can use a commercial device as long as you incorporate it into your overall design.

May students bring in their own beaker or graduated cylinder to test their salinometer/hydrometer.

No, they should use the graduated cylinder provided for the competition unless they can make a reasonable demonstration that a different container is needed.

What volume of water should each group be given for the salinity test?

A reasonable amount of water to expect would be 400mls to 800mls but this will be up to the local Event Supervisor.

What will be done to oversized hydrometers? Will they be given another container in which they can use?

Some accommodations will be provided for variations, but there is a limit to what constitutes a reasonable accommodation.

Would using a multimeter be considered as "commercially made meters" for salinometer?

There are commercial devices that can measure several parameters (pH, temperature, conductivity - which can be interpolated as far as salinity) that could be called a "multimeter" and would not be permissible. If "multimeter" means something else, like a generic electronic device, then it may be incorporated into the student built device. Students should be prepared to explain their devices.