Softshells are not a good choice.
For a start, all the fish in your pond will vanish.
Secondly, they are very private animals. They will bury themselves in the silt on the pond floor and rarely bask. Chances are you will not see them for long periods at a time.
Thirdly, their bite is horrendous, my Chinese took a decent chunk out my finger, and he is only 3" long. They hold on for as long as they can too.
Fourthly, if you do get them, get one. I've never understood the idea of keeping turtles together, they are not social (with the exception of maybe DBT or some Cooters). They may tolerate another turtle in their territory, but it can often end up with aggression. Softshells attacking each other will no doubt lead to death of one or both participants, since they don't have any form of protection.
Finally, they are better suited to a tank if you want to keep them. Outside they are vunerable to predators, and unlike sliders or painteds, if a bird/racoon or whatever decides to have a go, they will cause fatal injuries very easily. The vital organs are just a scratch/peck/bite away.
Paul
Also beleave it or not sliders and paints are allmost as vulnerable, unlike mature land tortoises, the shell of aquatic turts is like bullet prof glass in wich it only stops the first line of teeth, thats why they are more agressive and fast then tortoises. In a deep pond softys are safe, at the sligthest sign of danger they go for the deepest part were they remain burried. Also many softys are inpractical for tanks because the large poporsions they atain. 
As for our friend, he should tell us how big the pond is and that will determin the softys suitable for it.
If you want baby softys you need a tank, grow them for a few years and then to the pond
