I really don't think this will work out because the GT's will take almost all of that tank. There will be alot of confrontation and the jag might try and kill the GT's if they mess with it to much. We all know the jag will whoop the GT's @$$. You can definately try it though. It might work out. Really depends on temperment.
Hahahhaha,you really need a lesson in how Cichlids defend their nest???? Jag wanna eat babies,Gt's wanna kill jag just for being there,you do the math.
A lone Jag will be no match for a breeding pair of Gt's. That said, I think in a 125g everything will work out fine as long as there's plenty of cover. Providing them with some targets will also help your chances.