From my experience they do not need to be kept in groups. THey do just fine as long as they aren't kept with big fish that will out compete them.
They also will take a variety of foods other then live fish. The one I had (and my cousins) usually took this: live feeders of course, frozen silversides, frozen and live shrimp, frozen and freeze dried krill, and shrimp pellets (not the best food though...too messy). Your ropefish may not accept theses types of food readily though, it took my cousin's ropefish 2 weeks before it went after silversides. The one I had would go after worms and krill the first few days I had it. Sadly mine only lasted about 2 months before it got some bacterial infection out of the blue, after some research I found out it's actually pretty common among ropefish. Watch out for open sores, red spots, and the like. Mine started getting red sores and floating to the top, needless to say it died the next day and I had to move all the fish and re-do the tank.
Two more things;
Brine shrimp aren't even worth it, maybe as an occasional snack that's about it. Not only does it not provide nearly enough nutrition, but it probably won't reach it with your other fish in the tank. By having just one pictus- it'll be hard to get the ropefish to eat. B putting a small group your ropefish probably will either starve or be thin all the time. The aren't the best at finding food and pictus cats are notorious food vacuums of death.
And your pacu....
Your pictus will definitely outcompete your ropefish by themselves, but on top of it all you have the giant fat garbage truck of a fish in there too. Trust me, when you have a pacu in the tank NOTHING gets to eat. The one I had, when it was only a foot, ate a whole bag ( the BIG bag) of wardley algae discs, a stalk of kale and lettuce, (and various other things it decided to eat) EVERY 2 WEEKS.
YOur ropefish is going to have a lot of competition.