Agreed with above replies, a full grown severum requires a minimum of a 75 gallon tank, so if you have one in a 55g right now, you need to think about upgrading before the severum reaches around the 6" mark to ensure he/she can grow to their full potential size.
I have a GT with a severum, in a 125g tank mind you, the GT being only 4" right now, the severum nearly 7", time will tell how well it goes, like with all cichlids its down to their own individual temprement.
So in a large enough tank 120g tank + yes you could have EITHER a GT or JD with a severum, along with plenty of filtration and ALOT of common sense and constant monitoring of the situation, which can change very quickly.