Can I use Montage connected to an other Audio Interface instead of connecting it directly to my macbook?

Yes. Your analog audio outputs (MAIN L/R) would connect to your audio interface's analog inputs. Either 1/4 inch TRS, 1/4 TS, or XLR on the audio input side with the proper cabling between. Montage's MAIN L/R outputs would be either 1/4" TRS for balanced (better noise immunity properties) or 1/4" TS (fine for short runs). If going XLR on the audio interface side, you would use a balanced TRS-to-XLR cable.

Will it reduce flexibility?

Yes. This configuration allows for two channels of audio (output from Montage as input to your audio interface and eventually as inputs to your macbook). Left and Right. Using Montage's audio interface you would get 32 channels of audio output from Montage. The same Main L/R is delivered digitally along with 30 other channels of digital audio. Only Main L/R would carry the Master and System effects (and insertion). The other 30 audio outputs from Montage would allow for insertion effects only. You choose what you want to route/mix to each channel. Inputs to Montage are not severely hampered. Although there is some lack of flexibility there - it would likely not be impactful.

And If I connect it as stereo (should I even), is it still possible to do midi recordings along the way, to be able to edit it.

The effects are primarily stereo effects. The samples are primarily stereo samples. Yes - you should connect Montage as stereo in order to get the best experience. That addresses "should I even".

I don't know how you're going to connect MIDI - through the 5din cable or through USB - but either is an option. You can record and send MIDI from/to Montage through USB or the 5-pin DIN cable while also recording stereo through either your audio interface (2 channel) or through USB (32 channel).

Also doeo the midi recordings sound like how I hear and play it in the machine (velocity, sound moving from left to right, etc.).

Yes

MIDI doesn't sound like anything. If you've recorded the MIDI output from Montage properly (it's not difficult to do) then the MIDI would document all velocity amounts, note values, knob turning, etc. When you "play" the MIDI file back on Montage - then this instructs Montage what notes to press and knobs to turn. Similar to a piano roll in a player piano. Our piano roll would press the sustain pedal, etc. These are the "knobs" of the player piano. In order for the MIDI file to make sense - play back the same - you need to make sure to have either the command inside the MIDI file to recall the same Performance that matches the MIDI file - or make sure to first recall the right Performance on Montage before playing the MIDI file. The settings of the Performance are what would mostly cover that panning and other things respond as intended. This is not much different from any other modern synthesizer.

How you connect your MONTAGE to your computer will greatly impact your workflow. Using an external audio interface greatly reduces your ability record to your computer DAW. However, there is no need to use the 32-bus audio outputs if you are not going to take advantage of what it has to offer. If for example, you do simple solo piano recording sessions, there is no real advantage to having 32 audio bus outputs. But if your doing a full on music album, you might want to take advantage of what computer based recording has to offer.

The point is you do not need an external Audio Interface, as the MONTAGE has a 32-in/6-out Audio Interface for the computer built-in. This allows you to connect the 16 Parts of the MONTAGE each to its own stereo Track, or you can do advanced routing scenarios where you take individual outputs on each Drum sound, on each instrument Part, etc., etc.. It depends on what you wish to accomplish on each recording project.

The beautiful thing is you can use either method or both methods. The MONTAGE allows you to record audio at 24-bit resolution, and sample rates of 44.1kHz or at reduced number of simultaneous outs at 48kHz, 96, KHz, or 192kHz sample rates. As long as you keep a consistent rate for your project, you can easily switch audio interfaces mid project — many folks don’t realize you can record your musical instrument tracks using, say, the MONTAGE, as audio interface, then later setup and use a different audio interface to overdub (add) new tracks with a different audio interface. This is done often — there is no one way to work.

Why would you record using the MONTAGE’s multiple audio bus system?
You would do so because it is likely the most flexible way to use an instrument like MONTAGE. But it depends on you final goal.

MIDI is not sound. It is coded messages that document a musical performance. MIDI Recording will always require you have a MONTAGE to properly playback your data. You can’t just run over to your friends house and playback what you’ve done (unless they, too, have a MONTAGE!
Audio recording, once completed and “mixed down” (the art of taking multiple audio tracks and reducing them to a convenient stereo wav file)— which you can take to your friends and family and playback on equipment they likely own (any computer, smartPhone, tablet, or handheld music device).

Recorded Midi data is quickly and easily edited and allows you to “change your mind” before committing to an audio format (more permanent).

If you already have a another audio interface make sure it is an improvement on what you have already built-in to your MONTAGE (no need to downgrade your sound before you even get started) — I highly recommend that since you have both, learn to use both.
Do a session using your external audio interface…
Do a session using the built-in audio interface…
Compare your results!

Learn to configure and work with each piece of your gear, experiment, learn to master both!
You will discover that when you do, you don’t have to ask anyone, you will have a good idea about how to go about creating the results you need. Start easy, then work your way up. You can always get help here on YamahaSynth!