When I had my silver I had a 10in synodontis angelicus with it. Any of the suggested above would be fine aswell besides the RTC. I have nothing against them they just get way too big. Heres info on the syno. http://www.planetcatfish.com/catelog/mochokid/synodont/278_F.PHP
YOu could probably go with any cat you want as long as it cant eat the aro or the aro cant eat it. I love tigrinus, juruense, but there are alot more out there that would be fine.
I keep my silver with a juruense. They are all small right now but still get along. I would think that most catfish would get along with any aro since they occupy different levels of the tank. That is, as long as neither are big enough to get eaten by the other.
Some do; it depends on the species, so you'd have to research what diet best suits the species of cat you decide on. Just make sure the food will make it to the bottom of the tank before the aro eats it up. I have found that feeding the cat with the lights off helps, but then you don't really get to watch him eat which I think is part of the fun.