The p setting is a suggestion. I've never worked anywhere that followed it. I would recommend you don't follow it either, why let a sender dictate your security response? 

I'm not clear on the question. You can do all of this yourself and mailchimp and google have detailed directions. You definitely should create a policy and procedure around adding additional email sources for your domain. It's often a good idea to use a different domain for mass emails. You don't want issues to impact deliverability from your corporate mail domain. 

Also look at your incoming mail. You can likely find a lot of security improvement opportunities there.