A real Tank is of more use than a real priest, as for difficult content you may can pull out a healermerc or two, wheras a lackluster tankmerc cannot be supported with a second one as easy. If you choose a sk, you can almost complete avoid adds when needed, pulling manually, so cc is not a must have.
If you choose to add 2 classes to the team but one plugin, I would choose to play/drive with the sk and add the shaman with the plugin. I use a priest plus the plugin.
For my sk I don‘t have the plugin(yet), as boxing for my feeling works best, when „driven“ from the tank. For lazy times I have a kissasist set up for my sk, which does a good job.
If you add 3 classes, I would play the tank, priest plus plugin, cc plus Kissassist, i.e. Bard. My feeling is, my bard does a good job with kissassist.