In a closely contained environment like that a softie would not really be a good idea. The hard shelled turtles are likely to make contact with his shell surface and their claws can cut and scratch him this opens up the possibility of septic infections and/or fungus. Their requirements are too different really. Also a sand bottom in the pond with not be the best thing for your hardshelled turtles and a HUGE PIA to keep clean. Best to just leave the pond hard bottomed.
You have a nice mix of turtles right now and throwing a softie in would "tip the apple cart".
I have just finished my indoor pond and it is the same dimensions as yours but only 12 inches deep. I have 2 huge river cooters (that would most likely survive the winter months outside but I don't want to risk it), a partial amputee redbelly cooter and two rehab sliders (one blind and the other too thin to successfully winter outside) in it. It's nice having a mixed bag of turtles if you keep the right types together.
Steve