My stock seat (Hatchet) hurts after riding about 6 miles. Thinking of swapping out with bike seat like this. Anyone else have experience swapping out bike seats?
I’ve never ridden a unicycle with a bike seat, but I will say that bike seats like that one (Big, plush, squishy ones) are absolutely dreadful for comfort. Basically, the boney parts of your anatomy will squash down into the padding, forcing it to squeeze against the non-boney parts. Not brilliant.
YMMV of course 
That being said, I was going to recommend the Stadium saddle as my new favourite, until I realised the Hatchet comes with a Stadium as standard… So I’ve no idea what else to add.
There have been several posts on the forum about people using bike saddles. The general consensus seems to be that handlebars are required as there’s nothing to stop you sliding forward, unlike with unicycle saddles.
Give it a go, you have nothing to lose.
Other then sitting on the uni seat one of the primary functions is turning or steering. You may be able to tilt the nose up quite a bit but the Cloud-9 Cruiser Select Airflow CS Saddle would likely not be good for steering.
Quite a few have used bike saddles but most look more like this:
From this thread:
The bottom line may be what kind of riding you do and what hurts after riding too long.
Jim
I also like the Stadium saddle, but that is just my ar… Before changing, did you try different saddle angles? Actualy lifting the nose up, helped me. It looks unconfortable, but by doing so the back part of the saddle gets flat and you sit on your sitbones, which is way better on longer rides.
Give it few more tries… I liked stadium after week