All seems to make sense to me. The rat swarm 2 needs to make a soul(determination ) test DN4:3 or be frightened of Caith as well thanks to his Disquieting Presence. Whilst I assume frightened doesn't stack, I also assume creatures can be frightened of multiple things in terms of movement.
Does Rat swam 2 deal a flat 2 damage or 2 + S for a total of 3?