I think the deltas and unoffial megas are still in purity mode, you only have to look for them.
It said that somewhere, idk
Can I use both Purity Mode Charizard Y AND Delta Whatever?
What ARE you sure of?
Yes, you can.
Purity mode just partially remove them from sight, but if you want to use them trading can be a possibility or you can just simply switch the purity mode off from options.