Yes, you can halve or double the quantities of ingredients in each recipe to suit.
However, if you want to make half of the recipe of mozzarella, use the same amount of rennet as you would for four litres of milk, otherwise you may find that your milk will not set.
Generally we would recommend the full mozzarella recipe, as it works best in the full batch, but there's no reason you can't give it a go!