A cucumber should be just fine with lionfish. People really seem to have misconceptions about lionfish when it comes to harassing tank mates. Lionfish are predators, but they're ambush predators that wait for an appropriately sized meal to come close and then they gulp it down. If something isn't going to be view by them as food, then they are super peaceful fish. They won't harass or pick at any other inhabitants. My lionfish are the most peaceful fish in my tank in terms of getting along with their tank mates. A sea cucumber is going to be down in the sand and I doubt it would be an appealing meal for lionfish. They should just ignore it.