I have a Softshell right at 4 inches long that lives with a RES, a Florida Red Belly and a Map turtle in a 90 Galleon Tank and they have been fine for about a year now. Once they get bigger I can see some issues maybe, but so far no aggression at all towards each other. As long as they each have plenty of room to their selves and are well-fed, so be no issues. Just make sure you have some strong filitration going on, Maps like and good Current in the water and Softies need very clean water to flourish. .