The sev won't eat the other fish as long as they won't fit in it's mouth. Except the yoyo locah, they have barbs so if the sev tries to munch on them it could be the end of the severum, depending on how big he is.
A 30 gallon isn't going to be big enough for an adult severum.
And yes, severums can get HITH, though I don't think it's as common in them as it is in other species.