Question? Can you set different ringtones for separate contacts? Also can you set custom ringtones for text messages? These are probably stupid questions! Coming from BB I don't know anything about ios yet!
At this point and without jailbreaking, you must use the SMS/MMS tones provided by Apple... you cannot substitute the longer "ring" tones for SMS/MMS at this time.
Currently, you are limited to the sound provided by the app when it comes to fb and Twitter. Once iOS 5 is released you will have more flexibility with text and email sounds.
Most people, though, have expressed their dissatisfaction with the current set of text tones because they are too long and want shorter tones.