Just checked the Errata and it seems you are correct. That eliminates overlapping Spirit Guardians, and lets you stand in simultaneous fireballs. Interestingly, it also means an Evoker could cast Evocation spells against Evocation spells so the target automatically saves and takes the smaller amount of damage. That would be pretty resource intensive, but a curious tactic.Errata already solved that for you. Whether the damage from spirit guardians is radiant or necrotic depends on which was cast last. The most recent effect applies.
The damage does not stack. This is because stacking is deliberately restricted in the mechanics to prevent unexpectedly powerful combinations as much as possible. Seems WAI because not stacking damage prevents the powerful bard combo in your example.