Why not? But ones that dangle down look better. Watering can be tricky and annoying if it’s a plant that needs frequent watering. Depending on the circumstances, it can be drier up high so a humidity loving plant like Fern might not be 100% happy.

Technically, yes. But upward growing plants - those that are more shrubby or tree-like - will look awkward and it's hard to find hanging accommodations large enough to suit them. Instead, stick with trailing or draping or vining type plants. And make sure they are getting enough light.

OP, are you asking about hanging what is normally an outdoor plant inside the house? something like petunias or fuchsias?