Typically the max amount of damage displayed is 9999, but in reality the actual damage can go past this. You can see this if you get a scholar or a magic that can see your opponent's stats.
By using their ability and using an attack that flashes up 9999 damage display, you can see that the attack almost certainly takes more than that off their health.
With Gogo or Umaro in your current party, touch Equip or Status, then before selecting a character from the sub menu quickly touch the back button and the portion of the screen where Gogo or Umaro would equip magicite (assuming they could). You should now be highlighting a blank box, after which you can go into the esper equip menu to equip either character with your desired magicite.
Note that Gogo or Umaro cannot learn any spells even if equipped with magicite in this way, but they can still earn stat bonuses as normal by gaining levels.