Hi, since i'm fairly new to the speedrun thing, I'm not too experienced, so I have a question:

In my run I got a couple masks, so which are the best places to use Aku in case I get one?

I would use them whenever you lose speed for some reason, whether it be hit a wall, got hit with an item, etc. Alternatively, use it when doing one of the skips such as Papu's Pyramid. That's my advice at least.

Fair enough, but aren't there Aku only shortcuts like that one in Blizzard Bluff? Just asking here since it's kinda related lol

blizzard bluff is not aku aku only. no shortcut is aku aku only. Some are easier with aku aku, but none require it.

If you think Blizzard Bluff skip(s) are Aku Aku only, you need more practice :D