Is a guitar output in the same category as the empeg line-level rca out?

I'm not really sure what the specifics are. All I can tell you is what I know from experience in a similar situation. (This is electric guitar specific, so I guess it's more general interest than actually answering your question.)

One time, my amp died, but I wanted to practice with some volume. So I tried plugging it into my stereo via an appropriate cable. I found that if I plugged it into the CD inputs on the stereo, I barely got any volume. If I plugged it into the turntable inputs, I got reasonable volume, but it clipped a good amount. I don't know what this means in actual electrical terms, but neither one was ``proper'', whatever that means.

I have an acoustic, so it'd have to be a regular mike

This would probably be a little easier. You can probably find some code online that will determine pitch based on audio input. My first thought is that the sampling limitation of the mike input might be an issue, but the high E is only going to be about 300Hz, so that's probably not a big deal.

Also note that you can get a pickup for an acoustic guitar, if you're so inclined. (Note that not all of the pickups in that link are for acoustics, but many are.)