Because as far as I remember the item/companion is a singular element so that would require gm ruling. Though it could work since as a prince the rules get bendy around them.
Also metal and plant fusion elementals or golems sounds metal in the musical sense and I want one in the story I'm writing... How the hell am I going to do that?
Back on topic! On the other hand the choice of witch elemental will probably affect his qi development since that stuff is 'profound' and 'thematic' especially since the mount will probably develop qi as well... question dose his qi already have a flavor? A lot of cultivator novels have fire qi, yang/yin qi, lightening qi, dragon-phoenix-god-im-uniqe-and-special qi.... Point is type of qi is a common occurrence all in the effort to become a god. The mc is already a god-thing dose he have multiple types is he a dule metal/plant Pokemon cultivator?