I've been eating salads since about month 2 into my adventures in Ostomyland. I've never had a problem. I can't chew things very finely (dental issues), so some bits and pieces are poorly digested, but nothing has been complicating the passage of output through my intestinal tract. The only time I've had a problem from fibrous foods was when I ate beet greens (!) which definitely caused a significant blockage that had me pretty worried. So that's definitely off my list. (By the time I was eating those kinds of greens I'd forgotten the caution to try new foods in small quantities first and increase amount incrementally, to give self time to see how New Thing affects the digestive tract. I don't forget to do that any more!)
A long way around to saying, I agree with others here who've remarked that every body processes foods a little differently and you will need to experiment to see what works for you and what you can tolerate.
Regarding specifically what kind of lettuce: iceberg is mostly water, relatively little fiber, and is sparse on nutritional content. So if you want something that has good mouth feel (crunch), and is *easy* to digest, iceberg is actually the one to go for. Green and red-leaf lettuce and romaine are much more fibrous, also much more nutritious. They require somewhat more chewing, and you will need to see how that level of fiber agrees with you (or not). I notice when I do those more fibrous lettuces, that I end up passing more undigested bits in my output. Butter or bibb lettuce and various 'natural' greens like salad mix, dandelion leaves, watercress etc are more nutritious than iceberg and less fibrous than the three commonplace ones I mentioned above. So they might also be a good alternative.
Personally, I tend to stick with the iceberg and natural leaves, though I'll do one of those more fibrous dense leaves (like romaine or green leaf) if I, say, want lettuce on my sandwich. But for just eating a bowl of salad, I've had the best luck with iceberg, which digests down to nothing very quickly.
Good luck, and happy salad experimentation!