you can add any coral you want, but i would suggest slower growing corals due to the limited space in a 30.
i'm assuming from the avatar that you had no problem keeping montipora, so i would suggest mostly sps
and some lps
. no softies.
as for fish, 2 or 3 fish that don't require a lot of space. you have many options. do you have a cover to prevent fish from jumping? if so, i always loved the jawfish. the pearly, or if you can afford it, the blue spotted are awesome.