Nothing else. 30g is really not that big for a saltwater set-up. Boxfish will eventually outgrow the 30g. If you eventually move the box, you cna then add a flame angel and maybe a flame hawk.
from what i read about box fish and lions they can be put toghether. they both just sit and do nothing. altough they look cool. and if you have a 75 gallon tank that should be a good mix.