I think a lot of that comes from the fact that the Rebels get many of their ship from scrape yards. So Rebels and Scum sharing so many ships makes sense....Plus it would be nice for Rebels and Scum to have a cross faction ship like Imperials and Sum do, so something slightly different but still the same.
Rebels and Scum already have 2 cross faction ships, the z95 and y-wing
And the HWK! 3 ships
Pretty much. Or they get them from the used spacefighter lots.
Though in the case of the original lore of the Scurrg Bomber- Nym's ship its a prototype he stole or that he was able to get his hands on (I forget which).