Can you make passive mobs spawn in Superflat?

Slimes are the only mob that spawn (during the day) on my custom superflat world. I'm hoping to make other passive mobs spawn as well similar to default world generation spawn rates, is it possible? I'm using Flat world generator type with no block layers. There is a custom 1 block thick ground with plenty of grass blocks and biomes are set accordingly so it isn't all the_void biome.