thats fine, just dont make the con the biggest, they are aggro like you wouldn't believe and make sure you grow the GT a little before the con as well. My GT had to be taken out of my 75 twice because it grows too slow and the other fish just force it into hiding although they were all the same size at one point.
I had an oscar, JD, con, and GT for a long time. Plan on keeping it that way once my GT grows out more in my 29 gallon .
I would add the oscar first then the jd then the gt and then the con. since cons are so fast they will be alright being added in an established cichlid tank because they have an escape velocity. oscars are sluggish and oafish. Also they don't have teeth and all they do is just chase the other fish around. Good luck.