If the rats are male: Mangos contain high amounts of D-limonese (the same bad thing that makes oranges and citrus fruit a bad thing) and it's not just in the skin, it's throughout the body. So if your rats are male don't feed them it!
I wouldn't worry if you did this already but I wouldn't do it again. But considering your rats names are Penelope and Alice I'm sure they are fine lol. That is unless you named your male rats girl names.
I THINK it's fine, but I wouldn't feed them much. Best not to chance it in my opinion. Especially since it's bad for males of the species I kind of wonder if there isn't side effects for females either.