1. Angelfish are normally very peaceful fish, in times of breeding they can get aggressive. It is a good idea to only keep one pair of mating angelfish in the same aquarium, as more of them can lead to infighting.
2. It is generally a good practice to purchase the largest aquarium that you can actually afford. A larger aquarium will be less susceptible to quick changes in the chemistry, and will offer much more stability.
3. You should never add uncured live rock to the aquarium. Doing so can release harmful levels of ammonia into your aquarium.