small, skinny ferrets

Question
Hello,
I have two ferrets that I got from a woman who rescued them from a shelter. Apparently their first owners did not take care of them and then when they went to the shelter they were being fed cat food so they were in pretty bad condition by the time the woman rescued them. She nursed them back to health (plus they had some behavior problems) but when she could no longer care for them she gave them to me.
I have had these ferrets for about 4 months now (they were rescued from the shelter last january) and they are both very skinny. I give them ferretone and the high-calorie vitamin stuff every day but they haven't put on any weight. My boyfriend has a ferret that he got as a kit over the summer and she is already way bigger than both of mine. All three eat the same type of food. My ferrets are active and seem ok, but it really worries me that they don't gain weight. I estimate they are about 2 years old and the female is TINY- she is less than a pound and looks like a kit. My boyfriend's female is significantly larger than my male!
OK, so after all this rambling, these are my questions: Is there any chance that my ferrets will grow, or are they too old? Also, I was thinking of getting a kit next month but I don't want to risk it's health. Does this sound like a disease or is are they probably small and skinny due to their former malnutrition?
I am just so amazed at the difference between my rescued ferrets and the ferret my boyfriend got from a pet store. His is so robust and silky with a nice fat tail while mine are small, skinny, not so soft and with skinny little tails. Also, his needs a bath about once a week (her white bib gets a very yellow tint and she stinks) while mine aren't smelly enough for a bath more often than about once a month.
I read in one of your other answers that you have 20 ferrets. Have you experienced such dramatic differences among yours as well?
Thank you for your time!  

Answer
Hi Lindsey,
Are they just small or boney?  If they are small and female they may never get very big.  The supplements are great.  If they are skinny, try give them some heated up chicken baby food for a while.  They love it and should gain some weight on that.  Regular cat food isn't good for ferrets but mine have always been on Purina Kitten Chow.  It has everything in it that ferret food does and not one of mine is skinny by any means.  You have to mix the foods together to get them on a new one.  I assume you know that though.  
The supplements should make you new little ferrets hair soft.  I would give them some everyday.  I don't think they have any diseases if their stools are ok and they look healthy and are active except for their weight.  They probably won't grow anymore but should gain weight.  Stress can cause alot of problems with ferrets too.  They may be coming out of the stressful situation they were in and will get better and better with you.  
As far as bathing a ferret, I wouldn't bath them once a week.  It dries their skin out and can cause problems later.  Your boyfriend can get ferret spray that makes them smell good between baths.  The yellowing of hair is usually a hormonal thing.  

If you have any other concerns please feel free to contact me.

Mary