I just saw a Tweet from someone who just got a call from the casting director for The Bachelor to see if she wants to be considered. That should put some people mind to rest as to whether they cast the people specifically for each Bach & Bette. I think we all know the answer to that one.....nope. I think Jillian's last 5 were cast for her, but other than that, I think they pick the ones they need to make a good show.
ETA I think that some are picked with the specific Bach/Bette in mind, but in no way do I believe that all of Ali's guys were picked for her, just like other seasons.