Modes:
Dimming vs. switching:
Dimming uses variable voltage output. The higher the DMX value for that channel, the more electricity will flow. So, you can bring the lights all the way up to all the way down. Some type of bulbs don't do as well with some of the lower settings. Dimming is intended for ParCan lights using traditional incandescent bulbs, although there are new LED bulbs for the Par38's that I'm trying to get my hands on that are dimmable. DO NOT confuse this with the new DMX LED Par Cans.
Switching is basically on/off. Anything below a certain value(say, 128, the halftway point to 255) would be OFF, and anything above would be on. Basically, a "light switch". This mode is better for "high tech effects" like the Mystic, Sunray III, Vertigo, MiniSaucer and others that are not DMX capable and you want to use them. They don't like anything less than full current.
Dimming/switching mode depends on what you want. Since you're using Par Cans, dimming might be best. ParCans work fine in switching mode, just recall you're going to be FULL ON or FULL OFF in switching mode and you'll be fine.
Audio mode: Based on the dimmer pack you described, audio mode would make the dimmer pack cycle through pre-programmed scenes in the dimmer pack in response to typically a loud bass note(say, kick drum is ideal). Scenes are usually a channel or channels being in either a full on or full off position. Neat, but you don't want this for your application.
Auto mode: Dimmer pack cycles through programs all by itself, usually based on a timer-type setting. Again, neat, but not what you want for your application
Master/Slave mode: This is the "poor man's" programming mode, but don't take that as an insult. What it does it lets you use 2 or more fixtures and have them match without having to go nuts with DMX programming or even using DMX. One fixture is identified as a master. The next fixture is identified as a slave. Some fixtures can support multiple slaves but may have limitations, so read those manuals. Anyhow, using DMX cabling, you connect the slaves to the master in normal DMX-wiring sequence, and whatever the master fixture does, the rest follow.
Master/Slave mode should not be used in a DMX environment. It will break your DMX signal flow.
The FC400 doesn't care or KNOW what modes you're using on the dimmer pack. DMX is a one-way stream: It is sent from the controller, there is no response back from the fixtures.
My suggestion: you're using Par Cans, go dimming mode. How the FC400 gets from scene to scene is not something I know. Chances are you'll figure out how to make this work best for you in time. This is the fun part: learning your new gear. Have fun.