They research what can coexist. Some soft corals can be kept like a bouquet of flowers, most SPS
corals can be kept close together if you are aware of the different corals and how long their sweeper tentacles are. Same with LPS
. As far as the toxins they release (especially the softies) activated carbon and water exchanges are the only things that reduce those to manageable levels.
When placing a frag be aware of light and current requirements. Treat it like planting a tree with expectation that it is going to grow and fill in space. Some people drill their live rock and glue a plastic pin to the coral, so it can be moved if necessary.