This is tag is put up, just in case people mis-interpret/overread our conversation. Disclaimer: Following discussion are completely hypothetical; nothing official, confirmed nor has been suggested.
Yeah, if Grass and Bug were to merge in one club is a bit weird nowadays, as Bug-type isn't necessarily affiliated under the green nature anymore. While the Mineral-type merge examples, not all of them are truly mineral to begin with like Archen, Relicanth and Klinklang as you suggested. You've made valid points. :) There are also types that can't be really merge, like the Fire-type.
In case of partner Pokemon, that is actually completely up to the club owner. :) Before I took over the Grass-type club, the former owner allowed the members to pick any non-Grasses, for real. XD I erased those traces from the main thread, however. {XD}