A 120 is more then enough for a del. You could keep a couple upper jaw types together in a 120. Del, sen, polli, Palmas, gold dust, moke, ropes. 120 is perfect size for any of those. And all of those can live in harmony. Don't get a single ropefish. They are very social and need other ropes to feel comfortable. 3 minimum I feel.
Some drift wood some hardy plants and 3 ropes and a couple uj poly's would be a great tank. Just make sure it's got tops and that you keep very tight openings for filtration. Poly's can and will jump and ropes will climb out of the tank. If you keep it all tight you'll be fine. Also keep the water a few inches from the top as they breath air. What I have found in keeping these animals is that ropes like to play in flow and they will try to swim up stream so areas of flow need to be secure. Also if you leave enough space at the top of the tank at least 5" and give them a spot that is just below the surface of the water where they can kind of come out of the water they are happier and less prone to seek ways out.
I don't have the resources or the want/time to test multiple scenarios of why poly's jump but I think it has to do with how we traditionally keep them. Glass tanks with mixed fish or to many poly's and not enough places to hide always in the open, because if you give them hides they will hide. But then once they feel comfortable they not all but a lot will be out more often and much more happy with where they are.
