Once Apple released Pages v5.5 (now v5.5.2) for Yosemite, the last Mavericks compatible Pages v5.2.2 was withdrawn from the OS X App Store. Whatever version of Pages v5 you presently have installed on Mavericks is as far as you can go now. The OS X App Store will tell you that there is a newer version of Pages (v5.5.2) available for update — provided you have Yosemite.
Yes, you will get advice dialogs from Apple about an updated version of Pages availability, whether you are using Pages '09, or Pages v5. These are not requirements, and can be ignored.
As Peggy succinctly pointed out, you cannot open Pages v5 documents directly with Pages '09 v4.3 (index.xml file missing dialog) without first exporting to Pages '09 document format from Pages v5. If you open a Pages '09 document in Pages v5, and an autosave occurs, that document belongs to Pages v5.
Documents you cannot open with Pages '09 v4.3 should be exported to Pages '09 from Pages v5. Then pretend you do not have Pages v5 installed. Do not double-click documents, or by default, they will be opened by Pages v5.