Let's see. Old-school thief skills at a 9th level would indicate ranks in Hide, Move Silently, Listen, Climb, Search, Disable Device, Open Lock and Sleight of Hand. Now, these guys are pretty darn dumb, so we might want to consider giving them a racial bonus to one or more of these skills.
Although... they're natives of Pandemonium, so Listen is actually worthless. In fact, they might have racial deafness (no penalties for being deaf, immune to language-dependant effects) and telepathy for communication.