Oscars and sevrums are not very aggresive as far as cichlids go. Jags and flowerhorns are probably the most aggressive cichlids second only to P.dovii. 2 sevs, oscar, and a bichir is good for a 100 gallon. I wouldnt add too too much unless you have superb filtration and do vary frequent water changes. A synodontis would be ok in the tank. What are the dimentions for the tank?