Yes you always use the original config file from your version your running.
There is nothing else I know of other than what I have stated here. You are pointing your classifieds at this point to photopost database. If you PM me the admin login maybe I can see your classifieds but as I have priorly pointed out I can only see your gallery not classifieds.
Given the fact your gallery was upgraded with classifieds info this told me what you did here since the upgrade script is going to get the database and table info from your config file in classifieds and upgrade the database. This could only happen if your config file in classifieds had your gallery info instead.
EDIT.. I see you posted the same time as me
Your config file you where using had to be from the photopost not classifieds