If I'm currently in a plan with Cricket can I still be eligible for At&t Next?

So I'm planning on buying an IPhone 6s but don't have the money to buy it right away. I found out about "At&t Next" where you can pay in monthly fees. But if I'm already in a plan with Cricket can I buy the phone and use it with Cricket? Or do I have to join At&t?

AT&T and Cricket are different carriers. Cricket runs on AT&T's towers though.

Since they're different companies, you can do that. You'd have to join AT&T since Next is only for AT&T, not any other carrier.