I typically try to keep turtles around the same size since in some respects they are like fish, "if its smaller than me by an appropriate amount then i must try to eat it". Not all turtles will try this but even the veggies snackers dont turn their nose up at easy food.
I'd try to get a turtle aroundish 3 inches at least that should keep the yellow belly from pondering doing something unkind.
Also, keeping the yellow belly well fed should lessen the chances of problems as well.
What species of turtle were you looking to add to the tank, another Yellow belly or RES (red ear slider), souther painted, cooter, ect? Just wondering cause some species can be naturally nippy (softshells, mud/must turtles come to mind).
Hope this helps somewhat,
BTW what kinda filtration are you using, turtles are MESSY my three RES put a whole tank of fish to shame but at least i can do a 100% water change on the turtles and not feel bad about stressing them