Kuja's stats are fixed, they won't increase or decrease depending on anything you do. The only two enemies I know of that have variable stats are Valia Pira and Tantarian.

As for Zidane, it depends on how you equip him as he levels. These are the max for each of his individual stats:

Hp: 7509 (9010 with Hp+20%)
Mp: 494
Speed: 50
Strength: 83
Magic: 77
Spirit: 50
Magic Stones: 72

...but keep in mind that you would would to focus on one individual stat while levelling. He can get 50/50 speed and spirit, but he'll end up with strength in the low 60s if you choose to max out speed, and etc

Edit: I just checked a save where I maxed him to 50/50 Speed & Spirit, and his stats are 50, 59, 56, 50, with 6405 Hp and 359 Mp.