With all due respect to Green Hornet, compression is rarely used as a effect, but rather as a processor. The difference is that an effect is side-chained from the main signal in varying amounts while a processor is inserted into the signal path itself.
Green Hornet is correct when he says the left side is marked as the mono side of the RNC, but that doesn't mean it belongs in an aux send configuration.
My advice would be to punt the mg802 and put a little bit more dosh into an MG102C, which has 2 insert points, and an insert cable. An insert cable has a 1/4" TRS (or stereo) plug on one end and breaks out to two mono 1/4" plugs, one of which will be the send from the channel and the other the return. The 'C' in the name of
the MG102C is for 'compression', but the RNC is bound to be a MUCH higher quality compressor able to do a much better job than the built in Yamaha compressors. You will be able to insert your RNC properly on either or both channels and take full advantage of the 'Really Nice' aspects of it's compression process.