Can I add a new macbook air to my og Time Capsule?

So since I'm keeping my old Mac anyway, if I should need to retrieve an old document, I could retrieve from the old mac book pro, right?

Correct

And secondly, I will just set up the new Mac book air to use the time capsule as its backup source?

Correct. When you do this, you will have two Macs backing up to the Time Capsule. The backups for each Mac are kept in separate files, so you will only be able to access the backups that a given Mac has made by using that Mac to view the backups. In other words, you won't be able to use the new MacBook Air to access backups that your "old" Mac has made and vice versa.

Can I simply add the new mac book to it?

Yes, assuming you are asking about backing up the new MacBook Air using the Time Machine backup application on the MacBook Air.

......I would like access to the stored files......

You won't be able to access the Time Machine backups that your "old" Mac has made on the Time Capsule, but you might be able to access normal files and folders that may be stored on the Time Capsule hard drive.

We say "might" because there are no guarantees on this since Apple has added a lot of new security measures on new Macs that can make access to older files made on another Mac very difficult.

It is probably going to be be one of those situations where you won't know if something will work the way that you want until you try it out on your network.

If for some reason you want to transfer data from backups that were made on your "old" Mac over to the "new" Mac, you can use Migration Assistant on your new Mac to copy files from a Time Machine backup of the old Mac over to the new Mac.

Migration Assistant is located on the new Mac as follows: Finder > Applications > Utilities > Migration Assistant.

The Apple Support document linked below provides more information on this.

Move your content to a new Mac - Apple Supportsupport.apple.com › en-us