itch.io

The planned "character skins" feature will likely include most of those suggestions you mentioned, since it should let players set the appearance and sounds of specific characters (such as Baldi), so if you had downloaded a few mods with multiple unique Baldi skins included, you could have one Baldi in your custom floor use a default skin, and the others could use other skins.

And for any custom floors that use character skins from mods not included by default, there will be a message asking the player to have those mods installed before being able to play that custom floor. Which is why if your custom floor uses skins from mods in it for some of the characters, you should include the mod folders used in your custom floor when you distribute the floor's folder somewhere (such as on itch.io or via a link in a video's description, for example).

And put a message on the download saying what mods you need installed (if there are any) to play your custom floor.