it depends on the softshells size. they will stay burried if the substrate is sand, so then they will be safer. but really i wouldnt keep him with an rtc or aro for long. even as a full size he will get eaten eventually. but good choice in turtle, they are in my oppion the best (other then the fly river turtle) turtle for fishtanks that are full of water
softshells are always best kept alone. They are either small enough to be eaten by its tank mates or big enough to eat its tank mates. There is no in between. If you grow him out to be just 6" or so (should be within a year) he will take down a fish up to 3' long. Do not underestimate them. I have tried and both times they destroyed something 3x its size.
I would keep him alone. None of my soft shells have ever done well with fish, reguardless of the fish's size. They are active enough to keep the tank from looking bare anyway. What species is it? If it is a female eventually you'd end up having to move her outside eventually. Females of most species are massive.