Can I cherry-pick particular products within that collection instead of the whole collection?
No, we can only allow using collections to create the bundle conditions at this moment. This is because when some of the products are moving off the shelf. Our app may not be able to sync. Also, if there are many products, liquid may not able to render these cherry-pick products in a short time. We just didn't experiment with the performance yet.
As a workaround, you can create a custom collection to include these products. After that, you can fill in the collection as a bundle condition.