Like above posts say, all sovereigns are mages and all sovereigns are warriors, but not necessarily good ones. Sure your 5 int sov can cast a spell, but it's not nearly as effective as a 15 int sov's spells - and your 5 dex sov can wear plate, but it's not nearly as effective as a 15 dex sov's plate. Theoretically, it should be balanced - the spells of a warrior-sovereign with 5 int should be as bad as the defense of a mage-sovereign with 5 dex.
Now the numbers may need some tweaking, I'm not claiming the balance is perfect - but the concept is sound, you can do everything, but your stats modify the effectiveness of everything you do from swinging a sword to casting a spell. That modifier may need adjustment, but there's definitely no need for drastic changes like stat requirements on items/spells, when the stats are already determining how effective those items/spells are.
By the way, the system described below is the whole reason a mage-focused sov can't simply put on plate and have the same effectiveness as a warrior, it's worth repeating in case anyone missed it:
Right now, strength is a modifier for your attack and dexterity a modifier for your defense. 10 is the baseline, a 1.0 modifier, and scales linearly such that 5 is a 0.5 modifier and 15 is a 1.5 modifier.
A Character has equipment giving a total of +10 Atk and +10 Def.
If this character has a strength and dexterity of 10 and 10, he will have Atk 10 and Def 10.
If this chraacter is str 5 and dex 10 he will have Atk 5 and Def 10
If this character is str 15 and dex 5 he will have Atk 15 and Def 5