After the RTS matures it can become more aggressive than when it was younger. Even though it may get along with its other tank mates while young doesnt mean that wont change.
Dont get me wrong, they can coincide peacefully with tetras etc, but its not a guarantee when they get larger. Its a 50/50 chance. I had a Large RTS (4 inches) with cardinal tetras, and other smalelr peaceful fish and he kept to himself and didnt bother anyone.