This is kind of a loaded question, as it's a highly individual thing. Many j-pouchers can tolerate lettuce in moderation, and yes, for the first 4 or 5 years post takedown, I ate salads all the time and didn't have any issues whatsover. Unfortunately over the last 1-2 years I've had increasing problems with adhesions, such that I've had to limit my intake of certain foods, lettuce in excess being one of them.
Basically, the bottom line is, YOU are the only person who can really decide whether or not you tolerate any food, and so what it comes down to is trial and error. Absolutely, go ahead and try lettuce (or anything else you might be a little worried about), in small quantities. If you have no issues, you can gradually up your intake of those foods.
It would of course be super easy if there was one single magical diet that all pouchers could follow, all of the time; unfortunately it doesn't work that way.