well, as you have fairly aggressive fish, I'd go with achara or redtail catfish...both types stood their ground and survived against the flowerhorn..but, based upon your tank size in your siggy, you'd need a much smaller cat. Pictus are good. Mine survived after the flowerhorn, and my 28" achara adoped them 1 1/2 yrs ago as his "children"...never attempted to eat them....and, they are good cleaners.
blood parrots won't do anything. Severums can turn real quick. My old one beat up my old oscar so badly that he lost his one eye...and the severum was much smaller. Dempseys are gorgeous, however can never tell how they will turn. Just go to lfs, look around, get something that will stay under 8" and make sure it has a log or cave that it can hide in...and the cichlids can't follow him.