actually there is a story re: his past.. but i haven't read it yet... i am skipping several voulmes so i can read something interesting.. (when sousuke was ordered to come back and was "relieved" of his duty to guard kaname).. after that, i plan to read about his past,.. (that is if i have time..) i haven't read the 'ikinari FMP'.. i will surely check it out next week when i go to sapporo

definately there are differences between the novels and the animation.. what i like about novel is that it offers more information, details, etc. about the characters' emotions... so basically we can read about sousuke's confusion and "sadness".. (he was really really sad and depressed when kaname said "i hate you!".. kind of cute..)
and also there are two types of novels: stories regarding S and K's school life (eg. sousuke's date with mizuki) and action stores (animation stories). S and K's school life novels focus on their relationship and sousuke's efforts to "fit in" @ school, with friends.etc.. (very, very, funny...

anyway, i hope i answered all your questions..if i didn't let me know..
and i started studying japanese 5years ago by myself..and then to a language school.. my grandparents were educated in japan .. plus my dad speaks japanese. so i can ask him ^o^.. and i travel to japan quite often .. so i guess that helps alot ... and i am from the states...