Personally, I'd say no soap at all of any kind. If your ferret actually likes to play in water (many people who want their ferrets to, actually only tolerate water and not enjoy), then you can let them. Please don't have the water at some awful temperature, and I'd keep the soap out of it entirely. And if they don't like it or want out, then please let them out right away.
And here I would like to fully clarify, ferrets do not need to be bathed at all. Period. The idea of once a month actually horrifies me. The only time a ferret might need a quickie scrub, and with just warm water, is if there is something nasty on them. Yes, warm water can remove poop just fine. If it's truly something much nastier and stubborn that doesn't want to come off or can harm the ferret, a teeny bit of no-tears baby shampoo should do the trick.
No need for any soap at all in most cases. But letting them play in the water in the shower is not a problem
Ferrets: Contessa Kitties: Watson, Oskar DIP Sinnead, Vincent, Boris, Zeus and Athena, Willow, Mr. Frodo, Indie, Lucrezia, Judge, Odin, Miss Emily, Suki, Cody, Aristotle, Butterscotch, Frankenfurter. RIP Herne, Ligeia, and Mr. Stubbs
This new apartment I moved into has a walk-in shower. Alice will follow me into the bathroom sometimes and will try to go in while I'm taking a shower, she'll realize she is getting wet and will dart out to roll around in my bed sheets. lol
If your ferret likes water, I don't see any harm in letting them play around in it! Just make sure you don't have any product lying around that might be toxic, and keep a close eye on them! You might want to just put out a tray and let them splash around in shallow water instead.