For mono sources, just keep the pan control in the center, and both channels of a stereo mix output will get the same signal. For stereo inputs, you may need to use a simple Y joiner to send both channels of the mix output to a mono source.
We run our system in mono, and for stereo sources we send the two channels to separate inputs on the mixer. This is primarily because we use a lot of split-track sources, and it’s easier for the operator to use two faders to mute the un-needed channel. Thus, we only need to connect to one channel of the mix outputs. If you choose to do it that way, and your mains run in stereo, you will have to pan the two inputs right and left.
.
