Can someone who has completed this game already tell me if you need EVERY pearl in the game to beat it, or can I safely skip one (namely, the stupid pellet game one).

Having completed the game couple of years ago I barely remember there being any pearls. So, either they're all very easy to obtain or they're not that important.

note that I've not yet completed it...
People have completed the game without every pearl.
From what I've seen so far, you should be okay if you just get the ones in your normal adventuring. The pearls hidden about are not essential, but there is an end game bonus if you get all 88.

You only need enough pearls to get all the parts you need from Mammago's. I just finished the game with 12 pearls more than I needed, and didn't find all of them.

Just as ZamFear said - You need them for new parts to Your ship. I have old retail version and there was interesting bug - I wasn't able to find enough pearls for first ship upgrade! :D Fortunately I realized it is something wrong with the game and quickly found a patch for it. Version from gog.com is probably the newest one, so You don't need to bother Yourself. ;)
And another interesting fact - on official game site there is a ranking. When You finish the game it shows You a code, which includes all Your achievements - time, amount of pearls etc. After copy/paste it on the site You are a part of that ranking (You need to have Ubisoft account, though). So this is another thing You "need" pearls for.

Every pearl is needed in order to launch your plan for leaderboard domination. At one point I was 333. Doubt I'm that high any more.

Wow, I just checked my record (it is few years old) - 298 / 51660. If I good remember I was 1xx when I uploaded my code. But I didn't finish mini-game, which just bored me.
I tried just once to make the best result in the game (I mean - time, amount of pearls etc.) but I want to warn You - haste spoils an atmosphere of it, so play that way only if You like to achieve new speed-runs and things like that. I wanted to remember beautiful story and characters, not frustration of "losing few seconds here and there". This game deserves it. :)

Yea, rushing through definitely means you miss just enjoying the world but I still enjoyed playing through it fast. I too could never beat that minigame. I guess it only registers a score if you beat it.