I bought some Yogourmet yogurt starter packages and it says to use 5g per 1L of milk. However, if I am making a gallon at a time - is more starter actually required than it would be for just 1L?
Recipes call for a certain amount of starter to maximize the chances that your starter bacteria will crowd out undesirable wild bacteria.
If you use too little starter you will increase the chances that some random bacteria will win the incubation war. Since you don't know what you will get this can be actually dangerous.
I would recommend making an initial batch with the ratio the starter recommends and then using that finished yogurt as a starter for a second, larger batch. Since you would then have the entire first batch to use as a starter you can scale up your recipe for larger batches.
Yes, it is required. You have to ensure that the microbes you want dominate those that are still present in the milk, even after pasteurization/heating. This is achieved by adding a sufficient starting number and maintaining the environment (e.g., warmth) for their optimal growth.