1. get your water parameters to match mine in my sig line and stay stable. That'll work well for starter corals.
2. be sure you have enough light. T5 (high output fluorescent) will let you keep most things except fussy sps.
3. dip all incoming corals in an appropriate dip to kill parasites and predators. Observe softies for several days in a quarantine tank to be sure eggs didn't hatch and start noshing your corals. You don't want those in your tank. If you see little crawly things after several days, dip it again.
Outside of that, you're good to go. There's no such thing as too many corals if you've got the light and the water. They don't jump, and they're pretty self protective: if they close up or contract, run for the water tests and get the water back to normal with supplements. Do track your alkalinity---around 8.3; and don't get a stony coral if you don't want to supplement calcium: they eat it fast. Simple water changes will supply a softie with enough. If you're not sure, stick to softies; if they tell you it's stony, ask is it lps or sps, and if they say sps, don't get it until much later. Lps is hardy; so are softies.
Good luck.
