Welcome to GC! You've come to the right place.
There are a few Mizzou people on here, and they can answer your questions better than I can. Also, poke around this forum and the recruitment stories forum - there's a wealth of information. If you're so inclined, you can start your own recruitment story (we're addicts
)
Don't worry about going back to the sororities you visited during COB. You can tell them something like "I really liked your sorority, but I decided to wait until fall so I could see all my options" - they shouldn't hold that against you.
Is that green top translucent at all? You might want to go with something a bit more conservative. Also, be sure you have comfortable shoes! Looks like you're set for pref, but be sure you have some nice comfortable shoes for the rest of rush.
Good luck!
Edited to add: I should mention that I attended a school in the Northeast where rush was somewhat laid back and class standing wasn't really an issue. Things may be different at Mizzou.