They are usually peaceful with fish and yes deeper water levels are ok as long as the current isn't too strong and they have things to climb on and hold on to like you have already provided, because they prefer to walk under water then to swim. Check out youtube vids with them inside planted tanks with fish. They don't mess up the plants like other turtles either. The first pics you posted looked like a common sapper to me too but then I was sure it was a musk turtle. Very cure little guy and a nice find, hatchlings cost about $20 and you can keep several together, no problems. 20 gals is perfect for him and maybe a friend.