Congratulations on your new Montage... take your time. There are plenty of articles and tutorials to get you oriented:
Montage: Getting Started

#1 I have been using Performance mode on my Motif ES to make layers and splits. Is it the same on the Montage? Or what mode do you recommend to make splits and layers?

The Montage has but one mode, Performance. So you will use that for your splits and layers.

#2 How many splits/layers is it possible to do at the same time?

That depends on your programming. You can play up to eight Parts in a Performance simultaneously. A Part can be multiple sounds (depends on your programming).

#3 Is it possible to connect a MIDI or USB-keyboard as a second keyboard to the Montage and play sounds from the Montage? Just to extend the Montage 7 keyboard. And what keyboard/brand would you recommend?

The Montage is MIDI capable, use whatever second controller you wish. It connects via USB to computer or iOS device. We always recommend Yamaha, but you can use whatever you like.

#4 I would like to load samples, like vocals samples and stuff. Is it possible and how do I do that? And does that affect the number of possible splits/layers?

The Montage does not have a sampler. It can load sampled Waveforms that are in Motif XS/XF, MOXF format.

#5 How would you recommend that I set up my sounds with levels and so on, should I clear all banks and start from scratch and how should I set up the levels? On my motif I put the levels too high so when I needed to make a solo-level it was already max so I had to boost the master fader.

The Montage comes with some 2000+ Presets, the User Bank is completely empty, so starting from scratch is automatic. Set your levels so that it works for you... please be prepared to invest in a Stereo sound system to take full advantage of Montage best in class sound engine.

#6 Is there any good preset sounds for covermusic out there, like Final Countdown for example?
And regular sounds like nice piano/pads and so on...

I'm quite sure there is...

#7 Can you help me get the organ sound like the swedish band "Gyllene Tider"? Think it's some kind of farfisa organ.

Sorry, I never heard of them...

Thank you for your answers! It just arrived, and now I want to start editing it!
Where do I find the software for editing the keyboard? 🙂

http://download.yamaha.com currently has no software to assist editing performances on the Montage.

Some editing features were said to be in development by a 3rd party. The software available for the Motif-XF for editing is "compatible" with the Montage. Really, the Montage is compatible with this old software because Montage can import previous keyboards' save files.

"Editing the keyboard" is a bit nondescript. Perhaps you mean editing arpeggios or MIDI. In this case, there's a registration card that is included with your Montage for registering Cubase AI. In which case, you'll follow the directions on the card to get Cubase.

http://steinberg.net/getcubaseai

The first editing I would suggest doing on the keyboard itself.

Point of order: The Montage is not, strictly speaking, compatible with the software for the Motif XF. It is compatible with Voice, Waveforms, Arpeggios made for the Motif XS/XF or MOXF but their Editors do not address the Montage directly. It happens that the files they create can be imported to Montage, but they do not Edit Montage. Just to be clear. I'm sure the quotation marks around compatible were meant to mean this ... but just to be clear ...

To get started learning your Montage it is far better to learn the synth from the front panel. (Trust me, easier than software).

See this series of articles:
Montage Indepth Programming Series

Great! I would like to create some performances, edit effects and so on.

I have just tried it a little bit and it sounds amazing, I just don't know where to start like editing sounds and so on.
How do I edit waveforms, and find the category similiar to "Voice" part on my Motif?
Let's say I create alot of performances now. How do I avoid that it disappears if I import other files/sounds?
Are there are specific filetypes or any ways to import files that I should avoid to not clear all banks?