Hi Bellybutton,
The best way to tell how far apart to plant your shrubs is not by the size they are when you purchase them, but how wide they are expected to grow. Which variety you select will determine expected height and width. It's important to know that most plant labels give the expected mature size at 10 years of growth. It would be a good idea to look at several sites and go with the largest estimate you find.
With hollies it's important to know that crowding can lead to fungal problems. So it's best to allow some space for air circulation.
If you need sites on how to plant, mulch and water just let me know.
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.