Can a senegal bichir be really aggressive?

I bought an opaline gourami today and put into my 55 gal tank. My tanks been running for about 2 years. I have 2 plecos, 1 bullshark, 2 senegal bichirs, 1 needle fish, and a couple of . They all have plenty of room, but when I went to check on the newbie 2 hours later I found one of the bichirs eating him behind the house?! I don't know if the gourami died and then was eaten because they have eaten dead fish before, or if they purposely killed him. The gourami was at least 3 inches by 1.5 inch, and the 2 senegals are about 6 inches. Should I not try to add any gouramis, or look for bigger ones? I thought he would be too big for anyone to eat?