my friend has 2nd edition want help on power leveling

[JadedDM]

and if I'm going to go for the wizard, I'll go oldest for the wisdom and int boost.

Sure, just remember you'll suffer STR/DEX/CON penalties in return.

would tings be any different if I go cleric/mage vs mage/cleric?

They're the same thing.

I read barbarians can't read, but if you cross class to barbarian you don't lose your ability to read. or something like that.

There is no Barbarian class in core 2E. I think you're thinking of 3rd Edition.