Fluff wise your looking at 25 years of marine recruitment on Caliban before the fall.... So in theory there could of been a legion of fallen on Caliban.
Except the whole reason the 500 were sent back were to speed up traning because recruits were coming as fast as the Lion liked. We know recruitments were continuously being sent to the crusade based on the Master of the First audiobook. When the fall actually did happen only about half of them fell as there was fighting on the planet before the Lion arrived which we know from Zahariel's story and the Knights Errant stories, and most of them got killed when Caliban was blown to smithereens. Those surviving were also scattered through time as well as space which is why only a few pop up at any one time. Not to mention, you know, The Dark Angels have been hunting them for 10k years. If there were 10k+ black armoured Dark Angels wandering around, I'm pretty sure someone would notice.
I would like to see EC implement side missions of some sort for Chapters like Dark Angels, it could be a assassination or capture a player(Fallen Angel); a apothecary or librarian neutralizes him. Whoever is playing Chaos side, it can automatically nominate a player to be the 'Fallen' so if which ever side succeeds they get bonus points.
Again there are already Fallen in the game as a hero unit, though I do think DA should get bonus points or something of that sort for "capturing" or executing them. Though its our Chaplains that lead the hunt, rather than the Librarians. Librarians are jus there for the torture One Dark Angels Cpt blew his brains out with his own weapon after Ezekiel looked through his mind for a day and a night. I don't know of another legion who turns down promotions because they are afraid to have their minds read by Ezekiel (or similar).
Simple Solution: Create your DA char and slaughter your friends for the grace of the dark gods. I like playing RP...
Fallen wait do you mean those guys in black armour the fell down the stairs the other day . Wink wink
