All NL quality-of-life features, including pausing, are fine to use. All NL challenges (unless specifically stated otherwise) require just the normal solution, not any talismans.
You can prove your result by saving a replay, by pressing the "save replay" hotkey (U by default) either after the last skill assignment, or on the postview screen.
NL has automatic replay-saving turned on by default, so you may have a replay already -- look inside Replay/Auto/Lemmings_Redux inside your NL folder.