You are right, there can never be a "liaison" with "et". The "t" is never pronounced.
Whether there is a liaison BEFORE "et" is completely unrelated. It depends on the word preceding it. Personally, I wouldn't, but I'm not sure if it's an optional liaison or a forbidden one. I would tend to say forbidden, because otherwise it would be confusing: with the liaison, "intelligent et" would sound like "intelligente et". I'll be happy to hear from others! :)