One of my tank is a 125gallon: 72"x18"x21"(not including molding).
The occupants are as follows:
(1) 8" female orange midas
(1) 7" male jaguar
(1) 6" male jaguar
(1) 4" jaguar (sex unknown)
(1) 4" motaguense
(1) 6" texas (sex unknown)
(5) 3" white convicts
(4) 3" stripe raphael catfish (for clean up) (night janitors)
They all had been together for 4 months with no big problems other than flaring.
However, I put my 4" red devil (sex unknown), in the tank and it chases everything even the bigger jaguars, and fought for 1/2 hour w/ the texas cichlid.
However the 8" female midas will try to kill the 4" red devil non-stop, even w/ me trying to stop the chasing. So, now its on its own 10gallon tank temporarily.
By the way, as they get bigger some of this fish will be turn in to lfs for credit. Ill keep the nicest looking of the bunch as they mature.
So, yeah you can mixed aggressive, as you can see w/ my very aggressive fishes, it can be done. But it boils down to individual fish and trial and error.
But once you got the tank pretty stable. Its pretty rewarding to see all the flaring and some fighting here and there. Most entertaining of all my tanks, beats even my 150bow african show tank and of course my (7) 7"red bellies piranha tank.
Just my 2c