Yes. 

You do need to set the correct inputs re.: "Front" or "Back" for inputs 7 and 8 in the Fireface "Settings", but from what you wrote, you understand this.  For that many channels with a 192 kHz. sample rate, you should probably be connecting your Fireface to your computer with a Firewire 800 connection.

Yes. 

With the Fireface 800, you have ten channels of Analog to Digital conversion.

Yes, however...,

Just because your pre-amp has a rated max. output of +28 dBu, doesn't mean that it will be outputting that level all the time.  You have some sort of gain adjustment on the pre-amp - usually some sort of rotary knob - which you will adjust to suit what you are recording, so that an appropriate level is sent to the Fireface.  You have a choice of three levels of input sensitivity with the Fireface:  Lo Gain, where you will have a maximum input level of +19 dBu before clipping; +4 dBu, where you will have a maximum of +13 dBu, and -10 dBV, where you have a maximum of +2 dBV.  For your pre-amp, I'd choose the +4 dBu setting and be done with it.

Then, all you have to do is open up your DAW software, or TotalMix, and watch the level meters for each channel as you adjust the pre-amp gain knobs while your sound sources are performing a soundcheck.  I'd recommend that you set things so that most of the time, your levels are hovering around -20 dBFS, so you have lots of headroom for unexpected peaks.  If you wish, you can always raise the level later in your DAW or in dedicated mastering software.  With 24 bit recording (and 32, or even 64 bit internal DAW processing), you won't encounter any significant sonic compromises from adopting this approach.

This is a common issue, and not just with the fireface.

The only solution is to pad the output of your pre-amp if you're running it that hot. It's not uncommon to run an API 3124 to near-clipping to get that "API" sound for something like drums, but few converters can accept such hot signals. Occasionally (and really only occasionally) you want the "sound" of pre-amps running at their limit.

So an in-line pad (or a set) is very useful in these cases. Alternatives are solutions like the the a-designs level controllers (http://adesignsaudio.com/atty2d.html).

However, Frank's's advice remains important. Headroom is there just for that reason - best to maintain it!

While I might run the fireface at a +19 setting (I can never remember the high-gain/lo-gain terminology) in your case. In fact, since most of my own work is audio for post the +19 setting is the best match I can get to the -20dBFS reference level of all my production audio equipment. I use the same settings if I happen to be at work tracking with a quality analog pre-amp (especially older british designs.)

Thank you for sharing your knowledge with me

Regards