The biggest issue is that the softies will wage chemical warfare against the other corals. One way to combat that is to keep the number of soft corals to a minimum - just those you REALLY like/can't live without, etc. and, second, run carbon to pull out the toxins. As to long term, I think it's a matter of not relaxing that policy. I've kept softies and LPS together in a 29 for a few years, so I think it's doable. A larger tank would likely be more forgiving, but it's also a matter of how many soft corals you put in - more water volume but the same # of corals per gallon and that's pretty much a wash. With LPS it's a matter of placement to make sure the long sweeper tentacles don't contact any neighbors which can be stung to death. That's been my experience, anyway. Others may have different stories to round out the whole picture.