UKDaisy, did you get my PM?
I guess the bottom line is, you should be where you feel most comfortable. Even though your best friend is a member of one sorority, that doesn't mean that the same sorority is the best fit for you. You say you really like the older girls, though, so maybe things will be all right once the new girls get to know you. The flip side of that is, the new girls will be the ones you'll be spending time with after the older members have graduated.
I know you feel sort of snubbed (I would, too), but for your sanity's sake, try not to take it personally.

That sounds really cheesy, but let's consider for a moment that the group you wanted just isn't very good at COB. That doesn't mean they didn't like you. Personally, I don't think just telling people to "drop by the house" around a certain time if they want to works very well, because most people don't want to just march up to a houseful of people they don't know and ask to be COB'd. What we always did when we found out someone was interested was to invite her to dinner on a specific day so that she could meet the chapter. But different chapters do COB different ways, so take that into account.
As for formal rush, that may be the only way to get into the houses you're looking at if they're the ones who usually make quota during, or shortly after, fall recruitment. If you have any questions about rush at UK, I'll try to answer them for you. Good luck!