The consept is quite simple, actually: Your nichelings can only give birth in trees
You can just stick to that one and ignore the rest if you want, but here are some additional rules if you feel like it:
Start with as many nichelings as you want
Make them copies of the two default nichelings, but erase the names, change the immunity genes and give females wings
Don't unlock all genes (wings will be unlocked from the start of the game anyway so it's not game over if