28 temple guard are good. If you want more, I would stop at 30. I only own 20 myself. As to the disciplines on the slann, here are the ones that I use in high level games:
Focus of Rumination (extra die to each spell)-This is a must have
Focus of Mystery (Loremaster)-This is optional, but good to have if you want to make sure that you get that one spell...
Becalming Cogitation (No 6s for caster within 24 inches)-Another must in high point games
Etheral-Only if the slann is running solo or is in a unit of saurus
Hope this helps.