I'd need to look at the Slaan to answer the second.
I guess I've always assumed the GS could take wizard only items, it just seems better to use command-related items on him and throw the wizard stuff on Warlocks. It's not like the rolls are different. Looking again at the rules it states he can use magic like a Warlock, but not that he counts as a wizard. RAW I'd say no. RAI, probably, but why would you want to? Let me look at the relevent passages from the rulebook and see what I can find.
Edit: After re-reading the relevent sections of the rulebook, I stand by my conclusions. Pesonally I'd allow GS to take Wizard only, but I don't understand why you would. Spellcasting means being way in front if the line at times, and the GS is best in the middle (and slightly in front). With only one magic item per character, I'd take a Crown of Command or Orb of Compelling anyday over burdening him with a Dispel Scroll Scroll or Staff of Sorcery.