1. After considerations, I have decided to open the 1503 main plot line to AC1 characters as well to modern time characters, but with theses changes:
- The characters will have to be from the same tome, born into 1480 and so.
- The modern characters (Desmond, Lucy, Vidicq and co) will be native of 1480 and so Great Britain
- The AC1 characters will come from 1480 Middle East.
==> This mean you will check the real historical background to get the main lines of Middle East/UK government and situation.
Regarding background, you will have to adapt it: Malik and Altaïr could be assassin, and Altaïr Grand Master after Al Mualim treacherous scheme, but we will not keep the AC1 plot (the 9 murders, the quest for the Apple and such). However, you will need to find a way to explain why your character would be travelling to Roma.
And of course, you will need to get coherent between each other's background.
2. Rules 15 and 16 added to the forum rules thread
here