*moved to T-Mobile MyTouch Slide 4G forum*
Sure you can. It's kind of hard to find, which is why you might have missed it. First go to the contact apps and tap on the contact you want the costume ringtone for. Then select edit and you should see the option to set the ringtone there. Depending on your launcher, you might also have to hit menu again and go to options.
Note that if you want to use an audio file for that ringtone, all you have to do is put it in a folder called 'ringtones' on your sd card (no quotes). Same with notifications ... put them in a folder called 'notifications'.