I am planning to write something like crossover with Disney's movie "Beauty and the Beast" (but sadly, not here - my English is lying there, dying) and I struggle with some stuff. For example.

Population in prince's castle, changed into candlesticks, furniture - is there any ponyish counterpart?

Prince himself, in original story he changed from human to beast, then from beast to human again. What about Discord? what should I do with him, change, don't change?

And lots of other stuff, but I think I can do it on my own. I will ask here if I need anymore help, 'kay?
Any ideas?

If you're stumped on what to do with Discord, Robin McKinley's "Rose Daughter" or "Beauty" are both good Beauty and the Beast renditions, the former being one where the beast doesn't change into a human after the curse is broken. Reading those - or a least a summary of them - might give you some new fresh ideas that still co-align with the Disney version.