Perhaps you don’t need hair, except for some whiskers. Wet rats you could achieve procedural with some bump using a wave texture. A few clumps of hair on the large rat would be good.
You could use an Object Info node and use the random socket into a Color Ramp node. Here I used default black to white, but you can use dark to light shade of rat. This will pick a random position on the color ramp for each object using the same material.

The level of detail is you choice, and dependant on time. The larger rat is the only one I would spend any real amount of time on, as it is the focus. The feet could probably do with some more detail, claws for example.
The lighting looks good.
Great work so far @tanya53