It is hard to list what is working in which “branch” , so i think you will find “actual” version only. You can install/update some new versions, but you have to keep in mind that very next TurrisOS update will overwrite some stuff.
You can create your own local repository or you can add new repository url (and use it as any other else).
You can also create LXC container and install some linux,openwrt and benefit from latest collectd.
Yep, that’s the file, where you make your priority for install,uninstall actions over packages you have available via opkg.