A package set is just a list of packages in a file under /etc/portage/sets. You can operate on every package in the set at once.
https://dev.gentoo.org/~zmedico/portage/doc/ch02.html If you merged the packages by version they won't be automatically updated. This includes the case where they are part of a set (unless you leave part of the version unspecified). If you merged the slot, the packages would update to changes made in that slot.
