Will I be able to use UTH skimmed milk? Also do you have to use rennet to achieve a hard cheese? If so will vegetarian rennet work with the skimmed milk?

Rennet won’t work with any UHT milk, but will with any non-homogenised pasteurised milk, including skimmed. Skimmed milk does not make a decent hard cheese, neither does semi-skimmed, there needs to be a reasonable amount of fat in the cheese to make the cheese palatable, and to mature properly. A curd cheese can be dried out to make a hard cheese, not using rennet, but it is not a normal way to make hard cheese. Starters and rennet together are necessary for all well-known hard cheeses.