This Model helps with the concept of your lesbians clitorally penetrating minigirls. Man, what a wild sentence. Those concepts are already present in Pony apparently, but they usually don't work very well.

The main trigger is fairy clit onahole - I stole this from Pixiv ("クリオナホ妖精"). You'll also want to prompt minigirl and clitoral penetration, so: fairy clit onahole, minigirl, clitoral penetration

You can alo try large clitoris in negative prompt, because they do tend to become very large and rideable sometimes.