Can a Focusrite Scarlett 18i20 be linked with an Octopre Mkii via ADAT to simultaneously record 16 channels in Sonar X1 on a PC with Windows 8?
Thanks.

Going by the specs there shouldn't be a problem, whether your computer can cope With 16 tracks at once, and at what rate, is another matter.

Yes I believe you can, and I think you can also link to a link to a Saffire Pro 40 giving you the option of having USB or Firewire! Please contact Focusrite for further details.

It can up to 48K.  At higher sample rates, you are limited to 4 ADAT channels into the 18i20.

I've used an earlier version of the Octopre, the Platinum Plus along with 2 RME FF800s and an ADA8000 for a total of 36 analog ins all in one go at 24 bit 48khz using a laptop for live recording and mixing.
You shouldn't have any problems.

Worked like a charm but was a lot of work to setup. I didn't use Sonar though I used RMEs Digicheck for the recording and mixing

You can also use the SPDIF input on the 18i20, in addition to the ADAT.  I record at 96K, so I'm limited to 4 ADAT channels into the 18i20, but I also have a DBX 386 via SPDIF, so I have 14 usable channels.
 
Also, I decided to go with a Presonus Digimax FS instead of an OctoPre, just to get a variety of preamps.  They certainly sound different than the Scarlett preamps.  Along with a few outboard preamps, I have more colors available for whatever needs arise.  The convertors on the Digimax FS are also different sounding than the 18i20.