Stats and levels is all a matter of preference. I've beaten FFIV at level 45 and at level 78, it just depends how much effort you wanna put into it. The lower the level, the harder it's gonna be, but the higher the level, the more effort you have to put into it. Generally, I wait until Rydia has Meteo before fighting Zeromus
As for FFII, just equip two shields on all of your characters and fight for a while. You'll level up pretty nicely. Once I could afford all of the weapons/armor in the little town in SoR, I started to work my way through the castle.