Forogt to reply to Zapchu. hehehe silly me. :P
Anyway, yeah it's too late. The events following Iselia Ranch gets you to the second disc and cuts you off from all of the side quests from disc one. However, you start getting interesting sidequests that get you new costumes (I have 1 for Zelos and 1 for Presea right now) and some side quests do much, much more story-wise.
oh, and Sheena has E. Guard and Guard Plus on her lvl 3 and 4 gems respectively. These are both Strike-type EX skills and helps tremendously (as she needs every bit of defense).