Well, to answer your question, charizard_maa, I honestly never thought of that before now! But, I think they should only use their level-up moves, for these reasons:
- They have not been owned by trainers since their transformation, and do not find TMs in the wild.
- They were not hatched from eggs or bred in any way; Darkrai just transformed them, so there's no way they'd know egg moves.
As for reasonably using these level-up moves though, there's no levels in this RP per se, so... A Pokemon can use any moves it can learn,
within reason. So, for example, a Cyndaquil
can use Eruption, but its RPer would have to be reasonable and limit the use of such a high-level, powerful move drastically, or simply not use it at all until later. Use your judgement.
I'm going to go stuff this in the rules! Thanks for brining that up, charizard_maa!