They are quite slow growers! I can easily find a del nearby that is medium size. Now I want an oddball like a mokele from the Congo river basin that stays on or close to the bottom. And wouldn’t tear up plants.moke are really slow growers and you will be lucky to get 1inch of growth in a year, you won’t find any locally, most likely have to spend ~$140 shipping included from wetspottropicalfish. They have 6” ones. It’s probably half the thickness of your del, I doubt it could eat it. The possible problem you would have is more likely territorial aggression.
you could get a synodontis waterloti from AFD, or if their synodontis nigriventis (upside down swimming syno) are big enough not to be eaten