Unless you’re hosting VHDs and need maximum throughput (in which case use NFS), SMB is going to be the easiest to setup and maintain across those 4 platforms.
The Linux SMB implementation is decent and supports the latest version of the protocol (or close to, at least) whereas NFS in Windows ain’t so great and is a bit of a pig to get working in my experience.
I’ll have to have a look when I’m next in the vacinity but I’m pretty sure I have an APC Easy UPS on mine and it works out of the box.
Let me get back to you…
Update: It’s an APC Back-UPS 850. No doubt the instructions banged on about requiring Powerchute but I just plugged it into the Syno and it worked fine. You do need to enable UPS support on the NAS itself of course, from Control Panel/Hardware & Power/UPS, and set it to USB UPS.