can i create a backup cluster under same license?

In our license agreement, I don't see it explicitly forbidden or allowed.

However, given that

 - you're limited by database size and not number of machines, and
 - we were told we can have "Unlimited Test & Development instances (not to be used in production)"

things seem pretty flexible and I would guess you're fine.

Otherwise, I would consider it a Test/Development cluster until the moment I switch designations with the old production