Ferrets!

Question
I have two ferrets that had been bouncing around to different homes until I took them in. I'm not sure how old they are or how you can tell but I'm assuming they are still fairly young due to some of their behaviors. One of them, quite smaller then the second, is quite the nipper. I've tried a ton of things I read about on the internet and the only thing that works is the bitter spray and while I love that she doesn't bite it's hardly because she doesn't want to bite me it's that I taste bad. Not to mention when I use the stuff it somehow ends up in my mouth and won't wash off for about 24 hours. I have also done the whole scruff and firmly say no and not let them down right away but it doesn't phase them at all, as soon as I let her go she goes for my arms again!
Also, they are potty trained while they are in their cage (for the most part) but when I let them out they go wherever they please, not even necessarily in a corner either. I bought a playpen because they were ruining the carpet and digging in the house plants and squeezing behind the fridge and the stove. They love the playpen but its round so there are no corners and while they have a litterbox in there they only use it 50% of the time. When in there cage they have 2 corners that they have claimed for their potty spots but just last week they chose a different corner to go in!
They are just so wild and only half trained and everything I've read or heard doesn't really work for them and I don't want to have to spray myself with bitter spray just to not get bitten because the minute I don't have it on anymore she bites again!
Got any pointers?

Answer
Generally, I find scruffing and bitter apple useful in a very few situations.  I prefer time.  If these ferrets have bounced around, we are dealing with a lack of confidence, trust, and any bond with a human.  If you Yelp when bitten and remove yourself from the situation, this will let her know nipping makes you go away.  But, spend quality time with them hand feeding a few treats etc.
Regarding the litter box issues, welcome to the world of ferrets!  You are lucky they use a box in the cage!  You need to pay close attention to them when they are out playing and when you notice potty behavior, quickly place them in the cage.  Once they used the litter box, they can come out again.