I think this is the correct convention that's used in core.
Doesn't really matter, both are valid. Because String and Element are not in More, it has to be Core/String to indicate that it's in the Core package.
Also More/More.js provides MooTools.More (https://github.com/mootools/mootools-more/blob/master/Source/More/More.js#L25) so that should stay MooTools.More.
@timini you are right that Core and More use different YAML headers. The packager does accept both cases, still we will make a YAML cleanup next week preparing for the 1.5 release.
Thank you for your input on this!