MY TANK
The water is a little yellow right now, but its getting about that time to change it. No substrate, and I tried to minimize the stuff in the water because that is what the algae grows on. I have a couple bricks that they can hang on to sleep or just chill out.
For water level, I have always assumed the more there is, the more room there is for them to swim around and be merry. They pracitcally live in the water, so its not like they are going to drown. They should be able to turn themselves around comfortably in the tank. If they cant then its probably time to go bigger.
A good place to get babies is a flea market or a traders village. They are usually really cheap there. As long as the turtles are the same size they wont usually pick on each other. Some species are more aggressive than others, and meat in the diet will increase their aggression. My red-eared doesnt care for the veggies that much, but RES in general are known to be more carnivorious and aggressive. My bigger turtle loves veggies and is very mellow (but its not a slider, its a Cooter).