As far as I am aware, the actual firewall is a part of the linux kernel networking subsystem. Other 'firewalls' we are talking about are not firewalls themselves. They are just different configuration utilities for the only actual firewall.
In a perfect world you could use any of these utilities interchangeably. In the real world some of them can have their own configuration enforcement technique and several utilities enforcing their configurations can not be in an accord.
IMHO, the order of removal is irrelevant. And I am not sure that iptables have to be deleted. It looks like they are very basic and safe.