In my personal experience, I play VGC. I use PkHex because I am married with a child and do not have the extra time to breed or money to buy multiple cartridges to catch extra Tapu's. I create legit mons and I have no problem using them on battle spot. However, I am very careful and do not ever use anything that does not say "Legal" on PkHeX (green checkmark).
In any case, I think you should wait until PkHeX receives an update to start injecting transfer mons, especially if you are not experienced with creating legal mons, because at the moment PkHeX can not tell you if any transfer Pokémon are legal or not.