Yes, all modern ISIS cranks are interchangeable.
What other parts do I need to install them? I mean, is it possible to install them as I would any other crank, or do I need all that brake stuff?
No other parts. You will need a crank puller to swap them.
What size allen do I need?
My Nimbus ISIS hubs use an 8 mm allen wrench but I can’t guarantee that all are the same.
What about spacers? Do I need those, or can i use the ones already there?
Do I need spacers?
Yes, you most likely do need spacers. This is from an old thread, there shall be a press fit connection between the male and the female tapered spline of about 3 to 6 mm, normally around 5 mm. In practice this means, when you attach your crank arms by hand, there shall be around 3 to 6 mm space between crank shoulder and spacer (if not: take a thinner spacer!). Then you tighten it till there is no gap between spacer and crank left to reach the proper amount of press fit and then up to about 40 Nm which will tense up the screw enough so that it can not loosen itself.
You can buy an assortment of spacers for not much, https://www.unicycle.com/nimbus-isis-hub-spacer-kit/
There’s a bike shop not far from me that I might have put them on for me. Should they have spacers handy?
Get the combination isis spline crank puller and 8mm drive tool
I did a quick check and couldn’t easily fine one online.
Just take your unicycle to any decent bike repair shop.
I bought my nice isis spline extraction & 8mm drive combination tool from a local shop.
I’m sure you have some in your area.
What cranks are currently on your unicycle? They probably already have spacers.
My Nimbus II came with Venture 2 cranks and spacers. The newer versions have VCX cranks but they should be pretty much the same.
When I put the Spirit cranks on I reused the spacers that were already on there.
Usually there are spacers on the uni already and in my experience, they are most likely a reasonable size. The only times I had to mess with spacers, it was on old cranks.
If you haven’t got the material and tools (Old KH seatpost, saw and file) to make your own spacers around, and want to be 100% sure, getting a set like JimT proposed is a good plan. Most bikes don’t use spacers, so I don’t think they would necessarily have them around. You can also send an email to UDC, they’ll know if you should need different ones.
Also: Carbon spacers look sweet, but Carbon fiber is not the right material to use for spacers. Don’t buy them. They came on my used 26", and came apart, and took my bearings out at the same time. Not worth the 1 gram of weight saved…