As far as I know, there isn't a mite/lice that can move from rats to ferrets.
It's possible the marks on your weasels scruff are from rough play.
I know rat lice is little orange to red buggies, mites can't be seen.
I would think that since Revolution is safe for cats,dogs, and rats, that it would be safe for weasels too. I actually just bought some, and was thinking of putting it on my weasels because I do believe they have ear mites after trying many attempts with in-the-ear meds.
I would see if you can find a dose chart for ferrets for it tho. If a drop covers a rat, 3 might be good for a weasel ... I'm no expert tho!