Guppy science fair project?

Not if you want them to grow fast. My guppies grew best around 76-78, this link http://badmanstropicalfish.com/profiles/profile15.html states 64-84 degF as an acceptable range, but I wouldn't go below 70, personally.
No intentions on sharing the variables, eh?

I'm curious as to what your actual experiment is, it doesn't sound like your actually testing something out.

I think it a fine idea as long as you care for the fish. Guppys don't have to have a heater in the water. I know a friend that had some that lived out side all year long.

You should definitely try different brands of flake, try to have 3 fish on each food, that way you can kinda discount external variables

How long would they be in the tanks for, and why not just get a larger tank and divide it? You could keep up water quality, and it wouldn't neccesarily ruin the experiment if they cannot compete for food. Just set up sponge filters in each section, that way each compartment is equally filtered, and you get the benefit of having more water to work with. Lastly, get one of those fish food "rings" so that food doesn't float into the other compartments, so that everyone gets the same amount of food.

How about asking an LFS or big box store (Petco,Walmart, Petsmart etc) to help? They could potentially give you the following:

Long divided display tank (one lot of water, easily divisble, more stable, easier to manage).

Consistent water parameters so any changes in the fish can be attributed to controlled changes and not an individual tank getting a biological crash.

The fish themselves.

The food.

Pre-cycled filter media to get the tanks up and running quickly.

I'm not sure if the Science fair allows sponsorship, but it would potentially give you the equipment needed to make this more precise and more humane.