Corals of exactly the same species shouldn't sting each other, but corals of the same hobby-group and different species might. For example, all Micromussa lordhowensis (proper name of acan lords) won't sting each other, but favia species A and favia species B very likely will sting each other. Acan echinata will sting the hell out of Acan bowerbankii, but two Acan echinata will probably not sting each other. Probably.
Echinatas are extremely aggressive. Put them far from everything else. Chalices, same. Psammos have little to no sting. Montis will sting something that's directly against them, but that's about it. That applies to most SPS- they'll sting if something directly touches them or gets right up close, but most don't lash out. Mushrooms and leathers have no stinging ability, but leathers will get up to chemical warfare, and aren't necessarily great for a tank with stony corals. Run carbon.
With small frags of most things, you're fine as long as they aren't right up against each other. I'd put any LPS a few inches away from other things, unless they're micromussas (including acan lords, those are micromussas) or blastos. Blastos have a fairly strong sting but won't attack anything if not provoked, micromussas won't sting unless seriously provoked and don't seem to have much oomph to them.
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