Ah, I see then, the preludes are the remixes from the preview website awhile back?
EDIT: Quick Question: How to ability system in IV? Like Nocturne or DDS or Other? Can you keep all your abilities permanently or you gotta drop a skill when you get your list full?
Yes, Preludes and Hallucinations are the tracks that were on the website before the full launch.
It works like this: Every time you make a demon learn all its skills, a Whisper Event will happen. You can then pick one of the demon's skills to add to your MC's skill set. If a demon has a skill that is already on your MC, that skill will get a small boost (represented as a +1, +2, etc on the icon). If you're already full of skills, then you'll have to overwrite. You don't have a skill storage or anything, but to get a skill again is a matter of getting another demon with it and levelling it up.
By the way, skill slots are increased through Burroughs Apps.
I have never played this series before (preordered this one) and I have a quick question about demon fusing: The game gives the best fusions as a guide? Do you have to follow it? or are there special/rare fusions that you can only get by mixing and matching outside of the recommended ones?
The recommendations are pretty good most of the time, but they're very general. Often times you'll want something more specific to fit on your team, or a specific skill for your MC, so you'll want to fuse by yourself. Als, no special fusions appear in the guide, and they often lead to very good demons.
Keep in mind also that the fusion in this game is very different from most games in the franchise.