It sounds like the issue you're experiencing with the hair styles flickering in Unity might be related to how the animations are being applied or how the slots are being managed during runtime. Here are a few things you can check and try to resolve the issue:

1. Check Animation Keys: Ensure that there are no hidden keys in your animations that might be affecting the visibility of the hair slots. Sometimes keys can be inadvertently set, causing unexpected behavior.

2. Use SetSlotsToSetupPose: Before switching animations or skins, you can call skeletonAnimation.Skeleton.SetSlotsToSetupPose() to reset the slots to their setup pose. This can help ensure that no previous animation state is affecting the current one.

3. Clear Tracks: If animations are not being mixed out properly, you might want to use AnimationState.ClearTracks() to clear any remaining animations on the tracks before playing a new one. This can prevent any leftover animation states from affecting the current animation.

4. Check Skin Placeholder: Make sure that the skin placeholders are correctly set up and that the correct attachments are being used for each skin. You can also programmatically construct a skin at runtime if needed.

5. Debugging: Add some debug logs to check which attachments are being set during the problematic frames. This might give you insight into whether the issue is with the animation or the runtime logic.

If these steps do not resolve the issue, it might be helpful to create a minimal Unity project that reproduces the problem and examine it more closely.

If you are changing skins, you probably want to call SetSlotsToSetupPose() afterward.

It's odd to use a slot per hair when only one can be visible at a time. It's better to use a single hair slot.

Yes. Make sure you read this:
https://esotericsoftware.com/spine-slots

Slots 1) group attachments of the same type, and 2) decouple bones from the draw order.

There are no limits to the numbers of slots you can have.