Yup, he's too small but it might be OK once he's bigger:
"Overall, fairly compatible. Stinkpots typically mix well with basking turtles (cooters, painteds, sliders, etc…) once adult. Because hatchlings are fragile & more slow-growing than basking species, if you must raise stinkpots with baskers start off with a stinkpot at least 1.5” SCL, at least as large & preferably a bit larger than the basker. Stinkpots may be aggressive toward other turtles, particularly their own or very similar-looking species (could be an issue with Mississippi & 3-striped muds). Be especially watchful when mixing male stinkpots. Like any turtle individuals may be ‘antisocial’ & require solitary housing, & they have the jaw-power to do serious damage (like biting off a leg). If a stinkpot demonstrates open hostility isolate it quickly. Male stinkpots may sexually harass other species, which may become an issue. Stinkpots aren’t well-equipped for fishing & eat little plant matter, so they’re more compatible with fish & planted tanks than some popular basking species."
http://www.austinsturtlepage.com/Care/c ... inkpot.htm