Long-Winded Authors Note: Ok so I thought I better give a more detailed note on when these 3 fandom's take place in there respective timelines.
For me Roswell is the easiest of the fandom's to write - I am a Polarist (Mi/L) and I usually write stories centred on the Roswellverse but I am also a big fan of the crossover genre and my fave's are...
Roswell
Buffy the Vampire Slayer
Angel: The Series
Kindred: The Embraced
Highlander
Dark Angel
Vampire Chronicles (though that's not really a TV Series is it...oh well who cares)
And I'm not particularly fussed about pairings (what can I say I'm a shipper-whore) although I'm not really into slash. Enough about my hang-ups...moving right along...
Roswell - Because I am most comfortable with this genre this will be set after 'Graduation' Roswell - Season 3.18 but without the attempted military assassination, so the Pod Squad didn't have to flee Roswell and Max and Liz never married. I'm not sure yet whether or not others from the series will make an appearance. And although my Liz Parker is 18 almost 19 and you are wondering how she can work at a bar that will be explained in a future chapter. Also I don't really know much if anything about how American Universities operate so I just winged that part.
Before I start with the next two I have to admit that it has been a while since I have watched either of these shows so please bear with me as I slowly get reacquainted with the characters.
Kindred: The Embraced - Pretty much at the beginning of this awesome yet short lived series, Archon is still alive, Julian isn't with Caitlin and Eddie is both still living and the Brujah Primogen.
Highlander: The Series - This is the series I am have the most difficulty narrowing down due to its length and when I last watched it but Duncan will be the only immortal to make it into this story as a main character...although Ritchie, Joe and Methos could make guest appearances that could turn into co-staring rolls. Connor may also make a 'special' guest appearance.
Well that's all from me. Hope this helped answer any questions you may have.