Generally speaking, 10 to 14" apart is a good starting point.
To be honest, I'm not too concerned about 'chemical warfare' as much as I am aesthetics and future coral deaths from corals growing into each other.
10 to 14 inches may sound like a lot at first, however it's important to remember that SPS grow almost exponentially, a 1" x 1" -> 2" x 2" -> 4" x 4" -> 8" x 8" -> 16" x 16".
It's always important to visualize what your tank will look like ahead of time, 6 months from now, 1 year from now, 2 years from now and 5 years from now. Very similar to goal setting techniques for general life applications, the same visualization method can be used for your SPS systems as well.
Beatiful large sized colonies look much more natural and aesthetically appealing that simply having dozens of smaller colonies touching each other. I know with the amount of beautiful corals available to us, it's extremely tempting to jam pack the tank with cool frags. However, if you can resist that and have the patience to wait it out, the results will be amazing.