You got the instructions on the FAQ page? Make sure your guy adds the mixing resistors.
For your question about using the 5-pin and and the 1/4" stereo output at the same time, you can! Plug them in when you have the amp down of course, but it will work. You will be using exclusively battery power though, but you can use the outputs on the DS-5 the same as usual. Once you convert to mono, you can't use the headphone function anymore, but you ca still use the 1/4" mono out to something like a tuner, but again, if anything is pluged in the 1/4" output, the power source switches to battery only.