Megladoras irwini or niger both grow big and keep to themselves. As do Giraffes, all about 3 - 4 foot max out and are WAY better than TSN's and RTC's. I think anything at a foot and under would be threatened by an arowana.
do you want it to really eat poop or somthing? Think about it....?if eat too much poop it be toxic to them and end up dieing from living in it's own crap. (there will be just more poop.) how about a stick catfish? about 1ft.(they eat left over food.)(just get higher standard water filter.)
The best "scavanger" that I ever had was my geophagus brasilienisis. She would jus tgo around ALL day and sift sand through his gills. This was nice because it would turn over all the sand and make the poop dissapear.