There really aren't too many fully aquatic species out there that won't nip/kill fish no matter what the size is. Besides FRT the only other somewhat common, basically fully aquatic species would be softshells and most of them are very predatory with fish. Not to mention they can get BIG.
Adult size depends on the species of Softshell and sex of the turtle. Females get much bigger than the males. Softshells also need very clean water so you must have a powerful filtration system for them.