Yeah, got mixed up there. It's for skills rather than equipment slot. Either way, they are both insanely hard to get (when was the last time you got 3 copies of the same 5* girl, not even mentioning a 6* here), so it wouldn't be the worst idea if they include an item that would add +1 equip slot for any 4*, 5*, etc. They can even make it cost the same as skill items.

Alright then. Since someone else already mentioned the best way to collect Fireworks is by doing Advanced, guess I'll keep there until Ultimate Phase 2 is up. And as a plus, you already get a couple of crystals there.