I already know how to mix songs, piapro and stuff but now I'm wondering,...
The PVs , where do you get it legally?
and What MUST be in the credits? Yeah, just wanna makes sure I credit correctly,...
As far as I know, PV is downloaded via any video-grabbing browser extension/program. There are no official sources.
As for credits, there are no rules. As I have said many times, utattemita is not an exclusive club.
However, because you want to credit the original song composer, at the very least, link to the original song.
If anyone helped you with the cover (mix, backup vocals, video) then link to their channel/mylist as well.
You can look at other utaite's videos for examples.
Himeringo's Kanzen Hanzai Love Letter: There are links to an associated video (her cover of another song from the series), the original song, and voice/mix credits (herself).
Ryo-kun's Yoru mo Sugara Kimi Omou: Links to the original song, original composer's mylist, original artist's mylist, mixer, encoder, and voice (himself)
So, it can vary. Depends on how much space you want to use for crediting.