I have just decided that I will most likely be buying the 36" Nimbus Nightfox. I currently have a 26" Nimbus Oracle and a 20" no-brand, so this will be my largest wheel and hopefully my last unicycle (unless I decide to get a giraffe later). I do not like the saddle that the Oracle came with, and it looks like the Nightfox comes with the same saddle. I haven’t experienced any chafing, but it gets very uncomfortable during long rides. For example, today I got my new personal best distance. I went 20.83 miles on my 26" Oracle in 3 hours. The last 10 miles were quite painful, and there was no comfortable sitting position. I have been looking into getting a different saddle, and I think I have decided on the KH Fusion One (I am open to suggestions). Here’s where my question comes into play. The Nightfox frame is different than other unicycles. How does someone install a new saddle on the Nightfox? Do I need to get a seat post with the saddle, or is there no need to spend the extra money for a seat post? Does the KH Fusion One work with the handlebars that come with the Nightfox? Are the handlebars that come with the Nightfox good or helpful at all?
I can’t reply to all your questions as I have never owned a Nightfox, but there is something that might be interesting before changing the saddle: does your 26er have a t-bar? This increases a lot the confort on the saddle. As far as I know, the Nightfox comes with such a bar so you may find the saddle OK with it. At least, you should give it a try before tossing it 
The Nightfox does not have, nor require a seatpost. The saddle attaches to the frame via two of the front handle bolts, and two of the bolts normally used for a normal seat post.
The Fusion One saddle may take some getting used to, and I believe requires that you are adept at riding while holding the handles. A comfortable “transition saddle” might be the Kris Holm Fusion Free Ride.
A photo of the Nightfox saddle attachment points is below:
Based on this picture, the Fusion One seems to not be compatible as it doesn’t provide the four middle holes for screws.
The Nightfox uses no seat post and that limits the adjustment of the angle of the saddle. With the lower nose of the saddle that puts more weight on front and less on the back/sit bones where it should be.
This is what I did to adjust the angle. More photos and details of the saddle and handlebar, here.
The Fusion One needs a pivotal seatpost which has a single bolt mounting through the saddle (as used in BMX I believe) and does not have the ‘standard’ seat mounting pattern (as Maxence mentioned above). It may be possible to mount one of these with existing bolt holes for the handle and other potential holes that may be there – but more than probably not. You could always disassemble it, drill it, and bolt it on, but it is a saddle people either love or hate, so that might not be the best thing to do with a rather expensive saddle you don’t know if you like or not.
I have one lying at home to put on another uni, I can take a photo and let you see what the bottom looks like – I am away at the moment so it will be a week or so before I can do that.
If you really want to use a Fusion One saddle, we’re on a project to create a carbon fiber base for Fusion One saddles. It’ll have a standard 4-bolt pattern… And you’ll simply have to buy a Fusion One saddle, disassemble it without damaging the foam or the cover and put everything back onto the new base. It should cost less than 300€ 
I will just plan on not getting the Fusion One with the Nightfox. What comfortable saddles can be easily put on the nightfox? And does adjusting the seat to an angle make that big of a difference?
Jim T always comes up with a home spun solution, good job. I feel I’m in need of making a similar bracket, although maybe not quite as severe. I feel I could use a bump up on the front, it feels to me that mine angles down. The same saddle is not as comfortable on my night fox with 32 wheel as it was on my 32 titan. My night fox frame is the old style. I think they may of addressed seat position in the newer design.
In my experience, yes it makes a big difference in the weight distribution on the saddle.
This shows how the angle affects the weight in the saddle and even goes another step/method to put essentially all the pressure on the back with an optional front support thingy.
Would the Nimbus Stadium saddle be compatible with the Nightfox frame? I really don’t want to use the same saddle I use for my 26" Oracle. I feel like something a little flatter will help.
I would say so. As long as the saddle provides the 4-bolt attachment, it should work. That’s the case for the majority of saddles that are now produced.