LPS doesn't effect SPS that much as long as you keep the LPS tentacles away from the SPS. Keep in mind that LPS has some crazy feeding tentacles that come out at night that people forget about.
Leathers and SPS don't mix well, but if you run carbon and are on top of your water changes and maintenance, you should be fine. I've seen beautiful mixed reefs. My friend literally has a huge leather growing in the center of one of his monti caps without any issues, it just takes some extra care.
You really should try to find out the names of what you already have so that you can better get a grasp on what is required for them. Certain acropora sp. corals require different requirements.
Another important thing to consider is acclimating SPS to your tank, especially acclimating to your lighting. Take notice of what lighting they are coming from, try to find out how long they are under this lighting each day and how far away from the light they are. With having T5's, you should be okay. I have to be super cautious with this since I have LEDs and can easily fry an SPS that has been raised under T5 lighting.
If you're going to venture into SPS, I suggest investing in a coral dip such as Melafix or Coral ReVive.