For a new category, what if we get all 3 cat shines?

you mean giga bells?

no the shines where u get the cat mom her baby cats back.

Would that be post game or new file?

I think Post-game.

I think post game would work but u could do either

Could be a cool category extension come to think of it

Speaking of this category since you need to do the bully fight the new "Back off, Prince Bully" Skip will save time if it needs to be from a new file.

I think it should be a category extension that uses the same timing as Toad Brigade%

This has been suggested multiple times. If the mods have ignored it, there is likely a reason. It also would not be a Main Category. Main categories are generally reserved for runs that reward the player with credits, with no additional challenge. There are exceptions, however those are generally in the 100%/99% categories. (99% being the accepted 100% without the extra stuff). There is no reward for this category, as all you get are 3 shines. No extra star is added, no additional shines are unlocked, and you don't get credits.

It also likely won't be a CE because of how many times it has just been ignored. If you go through the CE forums, you'll see that there are multiple requesting this same thing.