Ogrork the Mighty said:
Where can I find the rules restriction on low Int and class levels? (non-animal companions)
Well, under Improving Monsters, the SRD notes that:
"Class Levels
Intelligent creatures that are reasonably humanoid in shape most commonly advance by adding class levels. Creatures that fall into this category have an entry of "By character class" in their Advancement line."
So there is no need to calculate whether a creature can advance by class - if they can, it says so.
Under Monsters as Races, the SRD also notes that:
"The separate table for Intelligence ensures that no PC ends up with an Intelligence score lower than 3. This is important, because creatures with an Intelligence score lower than 3 are not playable characters."
Under the ability score entry for Intelligence, there is this, which relates to the rest of the thread:
"An animal has an Intelligence score of 1 or 2. A creature of humanlike intelligence has a score of at least 3."
Does it say anywhere that a creature can't advance by level without an intelligence of at least 3? I'm not sure. It is implicit in the rules, but perhaps not stated explicitly.
edit: Don't really see it in the non-SRD stuff on leveling, either.