Re: Galciv III and most other games&software: if it works on Windows 7/8/8.1 without a fuss, it will work on Windows 10.

Caveat: there may be driver compatibilities for games for a few months, nothing critical, but you may expect some performance losses or instability for some games.  It's hard to get a full picture until Win10 is officially out in the wild, but from what I heard on other games, it's pretty stable, and some major studios are already using it.  It will bring DX12, but GalcivIII does not use it for now.

Re: Start8.  Stardock has announced Start10 (already available on their store, I think) to replace the product.  It costs 5$, just like Start8 (wich was the best 5$ I've ever spent)

Fences, I don't know.

EDIT: found this thread for Fences:

https://forums.stardock.com/460757

We have a Windows 10 box we have run GC3 on and it works. In fact about 2% of our current players are playing on Windows 10. But there is a big difference between that and real testing and QA.

We are waiting for it to be closer to Windows 10 release before we dig into it to deep (for now we are fixing issues on supported operating systems). But when it is closer we will certify it on Windows 10 and work with Microsoft and the driver creators if issues come up. GalCiv3 is a long term investment for Stardock, and we are excited for Windows 10 (so much so that Yarlan and I worked in the Microsoft booth at GDC this year showing off Ashes of the Singularity running on Windows 10).

I suppose you get that for free   looks good, maybe after win 10 comes out