A turtle pretty much nixes anything you've suggested.
The water level of the tank will need to be dropped somewhere from 8-12 inches. You will have to experiment a bit with the level to meet the turtle's needs, as I'll outline in the next couple of sentences. The turtle will need a land area with a heat lamp that keeps the basking area around 90 degrees F. This area will also allow the turtle to get the UVB light it needs from a separate light. They need the UVB for vitamin D synthesis. The land area is going to have to be large enough so the turtle can come out of the water to bask and have a cooler area for resting as well, around 75 degrees F, and it will have to be at the right height in the tank to accomplish these goals. You should be able to accomplish this heat gradient with a platform, either homemade or commercial. And before somebody says mud and musk turtles are 100% aquatic, they are not. 95% or so maybe, but not 100%. The end result, you've reduced your water volume by 30% to 50%, reducing your stocking capacity.
Turtles are pigs. They are messy eaters and they create a lot of waste. You'll need a filter that can handle a good sized load. Because of the water level, you can't use a HOB filter. Best bet is a cannister filter. But since you're sharing the tank with your brother, I'm assuming you're young and that may not be in the budget. A good-sized sponge filter with weekly 50%-70% water changes with a good vacuuming of the bottom of the tank should work and is the most economic.
The end result is you have a high bio-load from just one animal and you're limited in filtration choices. A second large animal in the form of an 6" sunfish is probably not a good idea, at least for now. Anything larger than 6" is just outright irresponsible fish-keeping in the reduced volume in the tank. Your best bet is a small group of five or six 2"to 4", fast moving fish like shiners or dace. You could try a single small to medium sized sunfish. Just remember the more animals in the tank, the more maintenance is required.
The turtle may take bites out of or even try to eat fish, or it may not. It may nip at fish fins or it may not. It may try to eat or take a bite out of a fish due to color, or due to movement, maybe one or the other, maybe both, or maybe none of the preceding. My point here is that every turtle is different. Every turtle has a different personality. That may sound crazy, but it's the truth. End result, well, you'll need somewhere to keep fish that the turtle is picking at if it shows that inclination.
Keep it simple to start with, expand as you become a little more experienced. You will enjoy your tank more and your pets will be happier.
No, I can't all my turtle know how actually came from working with a guy who ran a turtle rescue and rehabilitation operation. Although in reality he took any unwanted fish, amphibian or reptile other than crocodilians.