Is this a good science fair project?

Using gelatin rather than agar as the solidifying agent carries a major risk. Because gelatin is derived from collagen, a protein, many bacteria are able to use it as an amino acid source by secreting enzymes that liquify the gelatin by hydrolyzing it. In fact, gelatin hydrolysis is a test to help identify the bacteria. If you end up growing one of these species, your plate will turn to liquid and you won't see any colonies to count.

Agar is a polysaccharide and most bacteria cannot degrade it so you will avoid this using agar plates. You could still use the gelatin and see if it remains solid in preliminary tests. If it does then you can use it. But if it liquifies, you will have to go to agar. You can buy agar in some food stores (it is used as a dessert in some cultures) or go to a science supply company and buy agar media. Enough for a few dozen plates will be fairly inexpensive. Carageenan and pectin are also potential gelling agents although I have no idea about the concentrations needed but they are also polysaccharides like agar.

As for outcomes, you should count the colonies. The difficulty with only counting the dangerous ones is that it requires more extensive testing of each isolate to determine pathogenic vs. nonpathogenic species. You can't tell just by looking at the colonies what they are.