Filters, Musk turtles, and mixing types

I just wanted to give everyone a few updates, and ask a question at the bottom. Well, my Fluval 404 remains trashed, since some of the tubing seems to be missing, in addition to the little pump handle being snapped. But the good news is I managed to get my Marineland Emperor running. Though I can’t seem to find any of the inserts, and the only filter media it’s running are the two bio wheels. But it sure does do a pretty impressive job. I just had to dump bucket load after bucket load of water from the aquarium into the thing before it run under it’s own power. Now, a while back I mentioned wanting to add a musk turtle in with my DBTS, and got mixed responces. To those who said that they have kept musks in deeper tanks, and even with DBTs, I trust your opinion, but have opted to keep the musk seperate. For the most part because the 40 gallon breeder that I was keeping a baby water monitor in become available. The musk currently resides in that tank, and is using the Marineland emperor. Now my question is, though it’s somewhat off topic, are there any other species of turtle that would mix with a musk? Surely the red eared slider would be a canidate, but they sure can grow. A Snapper would also be a potential canidate I think, but it’s growth potential is even higher than the sliders.. I would like a turtle that wouldn’t outgrow the 40 gallon long tank.