I'm pretty sure there is no key you can use with the XML feed but I can assure you, you will not hit a limit. I've used the xml feed for a few years now for some projects and have run some tests and never got rate limited or banned. You can use a lot of requests on that endpoint and never get banned, but I wouldn't abuse it. You can go as low as 1 request every 30 seconds and still be fine. Depending on your usage, I would maybe stick to 1 request every 5-10 minutes.

Also, from my experience using it, I think Youtube caches the results server side, so if you keep requesting the same url, it will just send out the same page until they decide to refresh it on their end, which could be 5 minutes or 30 minutes to an hour. It always seems to change. I found that changing the url forces it to request an updated version much quicker.

So whenever you make a request, just add v={random-number-here} to the parameters, and use a random number each time.

Example:
https://www.youtube.com/feeds/videos.xml?channel_id=UC8yH-uI81UUtEMDsowQyx1g&v=4296133 <-- Random number

Example:
https://www.youtube.com/feeds/videos.xml?channel_id=UC8yH-uI81UUtEMDsowQyx1g&v=5342532 <-- Random number

Just to walk through this again to make sure you have done it properly.

1. Make sure you have a Google account.
2. Log into the developer console at: https://console.developers.google.com/
3. Create a project
4. THEN, select YouTube data API
5. Click create credentials on the right hand side of the screen.
6. Choose where you will call the API from
7. Pick public or User.
8. Obtain your API Key.

Optional: restrict your key.

I just walked though all of this and have been given a key myself.