any reason why a CZ858 would not work?
Any gun will "work"...
Of course how well it'll fit has to do with what kind of match you're shootin' and how it's set up... There's no one perfect set up for every possible scenario, but with the 858/SKS I see a couple of drawbacks.
1. These are... (and I'm gonna ge flamed by the red rifle fan-boys I know) military grade firearms. Sure they go bang every time, but I'm not convinced that any of these rifles are near as accurate or shootable as the most basic AR trype rifle. Of course here in Ontario that probably isn't a big deal 'cause we're not shooting beyond 100yards most of the time anyway. They have military type sights and from what I've seen most guys shoot military ammo outta these guns... Which leads to the next drawback...
2. Most SKS/858 shooters I've seen feed their guns "surplus" military 7.62x39.
Most likely 'cause it's cheap... Problem is, there are cases where such ammo isn't allowed... It's the MD's right to ban steel core ammo for the sake of steel targets/backstops... So, you may find that you're gonna have to spring for some costly commercial 7.62x39 anyway.
Then keep it straight... Mix up your rounds and punch a hole through a target or damage a backstop... Ooops.
3. The whole 5 round mag debacle... 'Sorry, but as MD I'm not going to limit everyone to 5 rounds because one or two guys are shooting 858's with 5 round mags or SKS's with 5 round strippers. The shooters solution to this is... 'Get awesome with your reloads
Of course.. There's always the exception. 'Seen it a few times where a dark horse shows up with what most would consider "less than idea" gear and said "dark horse" whips everyone butt
It's awsome to see, but rare.
All the above BS aside... If what you have is an SKS/858 and wanna shoot.. As long as you're safe... GO FOR IT.
You'll never really know what works for you by just sitting' on the couch flippin' through the LaRue catalog.