Just use the mono or stereo output.  the beauty of the 707 is that you can use the individual sliders as a mixer.

Hello, I have a question, and I thought I might find an answer here.  
The TR-707 has 15 outputs for the drum sounds, but I only have a
little mixer that has 10 inputs. What I'm wondering is, do you guys
use larger mixers for recording/live performance, or do you only
connect certain inputs It seems a little much to use a huge mixing
board for a DJ set, but maybe I'm wrong...