No miner yet - i've contacted the sgminer and claymore devs
All old clients won't work.
Decred has made stake version 3 active, all blocks have to have stake version 3 included in the header. All old versions of the mining software ignore that field and therefore hash incorrectly.
What will happened with unconfirmed transactions? For example with this https://mainnet.decred.org/tx/6c3adfcc769291e9cc724ae9106a862446444ab110890a6db02fba578468686d
No problem, they'll be included in the next block
Again a reminder: All old versions of the miner don't work - you have to use the new version