You could use it on an Aux, but I wouldn't suggest using it on more than one channel of the mixer and I would not use the return. You would use just the output of the compressor to send to the soundcard. An Aux channel effects loop basically just blends the effected sound in with the dry sound, unless your mixer offers control over wet/dry signals.
Why not use it on multiple channels via an Aux?
Because an effect such as a compressor/limiter acts on a single signal in regard to the attack, release and threshold you set. If you are sending a signal from one channel, the compressor will be acting on that signal alone within the parameters you set. Now all of the sudden you've got a signal coming from one of the other channels. The compressor may not even be done with the first signal (attack, release and threshold), so the second signal gets no compression.
If you require compression/limiting on all channels ... you need to either ...
A: have a compressor for each channel .... or
B: have a compressor on the master channel and record the sub-mix.
Use your Aux send loops for color effects such as delay and reverb.
~It all works out in the end~