Oh, these are very fun questions, and I’ve actually spent a lot of time coming up with answers, both for myself and for the stories!
Celeste’s curse would absolutely work on a nonhuman, she could easily curse a monster from the supernatural world just as easily.
This was actually something they covered in Supernatural, well after I had come up with the curse and effects on a human caused by proximity to angelic Grace. So, some spoilers for the final few seasons of supernatural to follow:
Alternate universe Michael, while controlling Dean, used his own Grace to power up monsters with similar results that I had given my own borrowers.
The monsters were stronger and more resilient. Similarly, any of my characters that have Celeste’s curse are very resistant to other spells. There are certainly strong enough spells that can affect them, but it’s like they have a thick skin to magical attacks or enchantments.
If we’re using Celeste’s curse, I would say all of a person’s forms would be affected by it. They will be stronger than an uncursed person, but she was very particular about making it so they can’t return to human size without her stepping in. She needs her little experiments to stay hidden, especially from other angels who might interfere and stop her.
The curse is mostly put on children because they are much more pliable and will grow into that strength. Adults can absolutely be cursed, but it’s more of a drain on Celeste’s strength, and she’d need to rest for a bit to recover that Grace.