How would a Ratling sub-class work in the game? You could theoretically be a sniper (Although a lot of the map is closed in).
However, the biggest issue would be melee, which is required by everyone, as you'll be half the height of most things and have few melee weapons to boot (If we go by the official Ratling Kill Team kit, Ratlings have only got shitty knives and are Strength 2).

I could see Militarum Tempestus, or even buffed-up servitors, but not Ratlings