Block generation on surface

I know this question has been asked before but having searched through every related topic I could find, I'm still left with the same issue. 

I'd like to generate rocks of different sizes scattered across the surface of the world. I have tried the flower and grass generation but it spawns too many near each other and just doesn't look very visually pleasing. I have also tried creating the rocks as a block and setting the 'Blocks this block can replace' to grass, flowers, etc. However, the rocks dont seem to spawn in the world at all.

I was curious if there was a simple workaround to getting a more even distribution of rocks across the surface?

Is there a way to limit the number of flowers generated in each cluster? That would also solve my issue (limit to 1 flower per cluster and increase instances/chunk).

adtl info. 

Rock block uses a json 3d model

I have the latest version of MCreator

MC version is 15.2

Thanks in advance!