The ES-8 is basically a multichannel digital audio interface crammed into a small Eurorack module. As such, it should function just like any other interface, save for the fact that it outputs CV and so forth as it's a DC-coupled device. In fact, some of us (myself included) use standalone DC-coupled interfaces from a number of years back. These would normally be considered "obsolete" for audio (except that they still sound fine), but given that some of them can also pass DC, they work great for this purpose. As for your DAW, it has the CV Tools device for using Ableton with something like an ES-8, MOTU 828 mkii (which I use) and such.
FYI, the smarter way to sync the DAW would be to use CV Tools to SEND clock information, since this will have much more stability than most modules offer. Taking your headphone from the mixer is also fine...it should have the same exact result as whatever's going into the ES-8, etc. But always rely on the most stable source for clocking, in this case the computer.