But then you can only record the stereo sum from the mixer, not so useful. Normally, you record each track separately, if possible, to allow post-processing of tracks and to be able to mix the song properly and to master it at the very end.

How high are your demands/goals on recording, mixing, mastering? So if you don't need a track of post-processing etc. and you think you can manage like that, then you could do it like that. It's not the usual way, but to simply record the stereo sum it should work.

If you intend to invest into a RME recording interface anyway, then I would really consider to buy one which has all required inputs and outputs and exclude the hardware mixer from the setup.

You will get better overall quality (sound, features, low latency) and more flexibility/options by this.
With TM FX it's also a much easier workflow possible, as you can store/recall routing and all settings digitally, very easy to use.

I couldn't and didn't want to work that way, because you couldn't adjust anything in the mix afterwards. Be it in the area of the level of the instruments, positioning in the mix and what not. In the DAW you could even consequently store/recall/adjust your typical inserts and settings for every instrument / track, create a recording template etc.
With RME you would have much better Mic Pres and also the technical data of the all I/O ports would be much better.
You're just giving away opportunities that are there. But everyone as he likes.

Regarding your question, I don't know what levels your mixer works with. Just make sure you don't overdrive the ports and everything will be fine.

But... just for the record (no pun intended). I can mix everything just fine in the DAW. As I've said, the mixer is only a device to have multiple instruments connected at once just to not have to unplug/plug an instrument everytime I want to use it.
Once the signal come from the instrument to the mixer, it goes to the AI, than to the PC and in the DAW gets recorded and I can do wathever I want with it as if I had the instrument plugged right into the AI.

Please allow me to give a little input.
@Lau
Replacing your existing PCI audiointerface with a Babyface is a good idea for improving soundquality and workflow inside the computer.
You will still use your mixer as before, no upgrade no downgrade.
One day you find out that a multi I/o interface is much more comfortable for the studio setup.
The main thing I want to point out is the mobility factor of the Babyface.
Together with an akkupack and an iPad or iPhone you can do recordings outside !
Put all in the guitar bag and record the guitar somwhere outside in the middle of the night at the river or in a park, in professional studio quality.
Having a friend with a second headphone and a Bass-guitar you even can do a recording session outside !
This is a kind of freedom you must check out !

Ok, now I got it. You use the mixer alone and you are recording every track single.
I thought that you might play together with frieds as you didn't detail this if I remember right ...

The only advice that I would give you is this. If you should think about replacing the Mixer entirely with a RME recording interface now or in the near future AND if you would like to pre-cable everything like you did it with the Mixer.
Check whether the BBF Pro has enough ports.

side note: mobility is maybe not the primary reason, why he choosed the BBF Pro (see analog mixer .. it's also not mobile).

Yes, ramses, the mixer is just for praticality. Of course if I intend to take the Babyface out, I will have to use it as it is. Then, invest in a preamp rack with adat is optimal.
The Babyface, as far as I know, will take 12 output and 12 input channels. So, more than enough for my purposes in this moment.
Thank you for your advices.

That's right vinark.
With the mixer by my side, I make fine adjustments on the fly without have to go to the mouse, minimize windows, select faders, increment volumes in the digital mixer... I make everything quite fast and is almost intuitive.
If I am playing over a track and I need the guitar to become higher in volume, just give a bump in the preamp channel fader on the mixer. If the next track is lower, same, fader a bit down. Same with EQ or other tools in the mixer. This without even touch the PC mouse. I'm constantly manipulating volumes and, I don't discard at all using the TotalMix, but for this details and little increments, the mixer by my side is quite useful and practical

Of course. To each his own.
I don't discard going full digital sometime in the future but, as I find having a mixer by my side quite useful, but not seeing that setup nearly anywhere (at least with a Babyface), hence my question.

Right...
That was just an example. I just meant several instruments plugged (whatever number) to have better access to volumes on the fly.
Nothing special.
I plan to use a preamp with adat in the future but, meanwhile, I would like to have all plugged in one place and with the flexibility of changing volumes and whatnot right by my side.

Thank you.