Universal and fire should stack and that wouldn't be bugged, potency and fire should not. Potency gives spellpower to each spellpower, like on morninglord's, universal is a separate spellpower that affects all your spellpowers. https://ddowiki.com/page/Universal_Spell_Power
Wow I didn't know lgs had that, as far as I knew burnscar and thrummingspark were the only enhancement spellpower items in the game lol... I really need to just look carefully at everything you can do with lgs sometime, and I should probably be farming ings and runes too lol.
"Exceptional" has always stacked with unique values. E.g. exceptional +1 int stacks with +2. Exceptional spell crit 10% stacks with 15%. etc.
WAI since mod 6. Doubt it'll be changed any time soon either.
Formerly known as Absolute-Omniscience, co-creator of the old DPS calc.