Is there a way to download a game with only a 30 gb allowance?

Can you download a game with a 30 GB allowance? Sure, if it's 30 GB or less and you don't do anything else that month.

Without using half your allowance? You still can, if it's less than 15 GB.

There's nothing special; if you've got a limited allowance then you need to make sure that whatever you want to download can fit within that allowance.

Yes, if it's a small game.

Depends on the game. Many game installs are smaller than 30 GB, but quite a few are larger as well. You will have to be more specific if you want better options than that.