Start out with cramming your head with info. I know you were given alot of info in this thread, but there is always more to learn. Also, just casually bring things up in conversations with your mum. Such as how rats are very clean, grooming themselves and eachother numerous times a day. Find out exactly what she doesn't like about rats. The tail, "smell", etc. See if there is a way you can convince her to change her mind on how she veiws that particular thing.