most soft corals arnt too hard to keep. never realy got to hard corals. but dont u think the equipment needed for corals would be a waste as they wouldnt apply to octo. light systems arnt cheap. matb better to try some delicate inverts such as shrimp. if there still around when u get your octo theyll b a good snack or sometimes if u have a good repore with yout lps theyll trade them with u for other supplies. though u never get back what u put into it.
Just as a note: categorizing corals as SOFT and HARD isn't that wise. Anyways definite no-nos for an octo tank are Zoanthids, Ricordea (mostly), SPS and LPS (pretty much any stony corals) and brain. They require too much light.