This might be a nightmare on the code side of things, but it feels weird to become a full player when cured as a baby zombie. When you cure yourself as a baby zombie piglin in pandemonium, you become a baby piglin, so I think it should be like that.
It would probably function like the other baby mobs.

I also have seen a couple suggestions asking for negative effects when you possess a mob for too long. I also think that those should be in place because once you have armor, there is not many benefits to being a normal player compared to a drowned or a zombie.

To summarize ideas for Pekhui compatibility:

• When you possess a creature, your Pekhui attributes are inherited from the possessed entity's Pekhui attributes.
• When you unpossess a creature, your Pekhui attributes are reset to before you were possessing the creature.
• When you cure yourself back into a player, your Pekhui attributes are copied from the possessed entity's Pekhui attributes, possibly with some modifications:
  • If you are a baby zombie, you become a baby player (multiply pekhui:scale by 0.5) upon being cured.
  • If you are a giant zombie, you become a giant player (multiply pekhui:scale by 6.0) upon being cured.

As far as I am aware there is a precedent - when baby villagers get struck by lighting they also grow up instantly to become witches. But I guess it could be an interesting compatibility feature when Pehkui is installed.