As you add more outboard, a patchbay really helps things. Mixers w/o inserts are like ... well, I shouldn't say on a family site. Same w/ preamps in an interface. My TC unit doesn't have inserts which is a bummer, since the preamps are actually pretty good.
As Mike says, you need another tubey compressor for stereo work. You can run one side of the stereo through your single channel at a time, but that introduces problems of its own. Still, it has been done. You can also use the EQ for some gentle shaping, while using software for more precise correction.
I've never heard of the Alesis as a mastering compressor. It is good for effect, pump and grind, but most music styles need something a little smoother for mastering. You can definately use it to run some stereo tracks out in back into your interface as Mike describes. I'd hook the channel strip directly into the interface and skip passing through the mixer unless you need it.
Another place where a patchbay comes in handy. See what you've gotten yourself into? The costs never stop.
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