as well as a number of the outputs from the mixer, to each split off to other devices (headphones, SD audio recorder, speakers, PC audio input, PC audio output, Mac audio input and output).
If you don't need separate volume controls, you can simply connect the inputs of a few devices together with [u]Y-Adapters[/u] (splitters). Or, you could make a patch-bay with several RCA jacks in parallel.
The general rule is - NEVER connect OUTPUTS together. (That's what your mixer is for.)
But, it's generally OK to connect inputs together. For example, you can connect a few amplifiers to a tape player or mixer. The load impedance drops as you add more devices, so if you connect too many devices you will start to affect the signal. It depends on the output & input impedances, but I'd say 5 line-level inputs to one line-level output should be OK.
Headphones need a lower impedance output than line-level, so headphones should be connected to a headphone amplifier. On the other hand, headphone voltage is approximately line-level, so you could connect many-many line-level devices to a headphone output. (The headphone output on a laptop doubles as a line-output, but the line-out on a desktop soundcard is not always rated to drive headphones.)
If you need separate volume controls for a few devices, you can build op-amp "amplifiers" with volume controls and a maximum gain of one. These should have an input impedance of around 100K (which is generally going to mean a 100k volume control at the input of each.
PC audio input, PC audio output, Mac audio input and output).
Be careful with your signal flow... It sounds like you might be making a feedback-loop with the computer output feed-back into the computer input through the mixer.