I can't decide what my Meowstic should know.
It already knows:
Is Shadow Ball better than Signal Beam?
Id say Shadow Ball.
Your Meowstic already have Disarming voice which counters dark. If you go for signal beam it will be not very effective against Ghost and it will be super effective against dark which is useless if you have disarming voice. and you have Shadow ball so you can counter ghosts. You can also change disarming voice for signal beam (recommendation) so you can counter dark with much more power. For fighting types, you already have Psychic so its useless.