First off, do you have any skinny breakers in the panel yet? GE makes "thin" breakers (the part number will have a P like THQP120) both in single and double pole styles up to like 50A. You may be able to replace some of the thick breakers with these to get more positions.

But anyhow, if you are set on the subpanel, no you don't need a breaker in the sub. If you're putting the new panel right next to the old one, it's not even going to be an issue of convenience. They call these "main lug" panels. That said, if your borg only has main breaker panels, an extra breaker won't hurt.

Nearly all panels are rated for either Cu or Al wires.

For convenience, if you have a GE Q-line panel, I might put another one of those in. That way you only have one style of breakers to worry about. Otherwise, I'd take a look at what the local borgs (or wherever you are going to by) stocks for breakers and such and get a panel that you can easily and locally get the devices for. It's somewhat of a regional thing. Some areas tend to Square D, others to the GE, or Eaton CH or BR panels. All of these will just be fine performance wise.

I didn’t know about skinny breakers. That sounds like a great idea, much better than a sub panel.