If I understand your question, you can hit with multiple mana reducing heros in one turn, so if you hit with both Li Xiu and Azar in one turn you would reduce all enemies by 20% (li), and one enemy an additional 25% (Azar).
The thing you need to watch is that some specials will overwrite others. The thing to note is if they use the same symbol they will overwrite each other see here: 📑 Status Effects (Buffs and Ailments) and Stacks - Dead topic see link in post #190
As to your second question, if you buy extra roster spaces or extra teams you keep those permanently.