you should be able to use inputs and outputs any way you want, even in some weird way like input 1 & input 3 in a stereo bus.
While I hope there will not be any problem in routing from a sequencer (I will mostly use Logic), I see that the dspMixFx console only allows four stereo Mixes (busses). By reading the diagrams on the back of the manual, it seems that these busses are hardwired, as if you couldn’t choose separate mono outputs.
You should have a look at the UR series specifications
Unfortunately I don’t have enough data, or I’m not skilled enough to read the data. The useful information is that these ouputs can work at max. 40 Ohm (versus 75 Ohm for the Line Outputs). They can output 75mW per channel, but how does it translate in dB? According to this useful converter (http://www.sengpielaudio.com/calculator-volt.htm) it should be about 18.7dBm, that should be the same as the Line Outputs (that is given in the specs in dBu). So, about the same level with a different impedence: how does it translate in the real world?
The RME Fireface 400 can use the Phones output as additional (unbalanced) monitor output; as the Steinberg’s, this is a low-impedence output, but RME declare it to work (and I could verify it can work well).
I would be glad to hear some clarification from someone that can read the spec tables better than me, or has done some tests.