PrinceOfFacade
Ghost-Type Master
Yeah they would have brought it up by now, if that were the case.
I feel simply that some Pokemon will be limited to certain Tera types, some being more common and some being more rare.
Like maybe Eevee can become any of the Tera 18 types, due to being Normal type, but maybe Tyranitar is limited to only certain Tera types,
I don't think there is a limit. I think any pokemon can have any Tera type, and it's all at random.
The fact that we have Tera Blast pretty much explains it. The move ensures a pokemon can make use of any Tera type.