A resident of a dwelling that houses a number of occupants, especially a person confined to an institution, such as a prison or hospital. (The Free Dictionary, n.d)
Because it is a prison, the inmates will all be wearing same uniforms just in different sizes. Although for races like the robot race it will be adjusted a little, maybe they could have an imprint rather than a uniform. And the Alien one would be modified so that it fits the alien.
Some inmates would look different because of tears and burns in places etc (So it all don’t look the same.)