How come black couldn't turn into an ordinary super saiyan right after stealing Goku's body?
Why Mai was a child in Trunk's timeline?
What exactly is super saiyan rose and why is his hair pink?
1. Stealing a body doesnt unlock its full potential right away. It apparently can happen that you need to adapt to the body and learn to unlock its potential.
We have also seen this with Ginyu stealing Goku his body in DBZ.
2. Mai is an adult in (future) Trunks timeline.
In the mainstory timeline, something with the dragonballs caused Mai her body to be turned to that of a kid, even though her mind is not that of a kid.
3. Super SSJ rose is this color because Zamasu a god, is in a non-god Goku his body.
Therefore his color is different.
Zamasu said that he was curious how mortals looked clad in god ki.
He said this when he saw Vegeta transform into SSJblue.
Meaning Godki doesnt look the same on gods and mortals.
Samasu his aura was always blackpurlish so that probably gave birth to his SSJ transformation color being the Roseblackish color.
It was likely inspired by the black rose (you can google that).
Which we see in both Zamasu his name (Black Goku) and in his color Rose.