These are 3-month old seedlings, sown around September 2015.

They have roughly the same size and number of leaves but their stems vary greatly. I want to trim them and get a bushier look. Would I treat the short stems and the tall stems just the same way?

I'm aiming at keeping horizontally-laid leaves to create a "shelving" effect just like the fungi growing on a tree-trunk. It is aesthetic, as well as more efficient in capturing sunlight and not too crowded. Many vigorously-growing pelargoniums create this effect. Reckless pruning creates disordered leaves and the bottom leaves do not receive enough light and then turn yellow.