You can change the chance, set min and max floors, and set floor multiples for each in Generic Mod Config Menu or the config file.
You can set specific spawn floors for each by editing the config file.
Floor multiples means they will spawn on floors divisible by that number (1 = spawns on every floor).
Technical
Requires SMAPI.
Implements a Generic Mod Config Menu interface to change config settings in-game.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.