Some softies can touch each other just fine, but its usually a problem waiting to happen if you put a softie too close to a sps or lps IME. Some lps have nasty stingers they put out at night that can reach a good ways, some sps can be nasty as well. Research is the best place to start, followed by a little trail and error, just keep a eye on everything. Usually more distance is better, because that small frag will be a big colony someday...