Just an FYI that the faction called “According To Plan” [ATP] are not pirates but bounty hunters, you should therefor not assume they are friendly!
Hm, they told me, they are Pirates for sure. Why you think they are Bounty Hunters? I guess both are not friendly 
Do any pirate factions actually collaborate besides SyP and SWP? I honestly never heard much teamwork with other factions, usually because they’re smaller groups.
Monin offered a bounty on me. One of ATP said they would take it. I pointed out that pirates weren’t supposed to attack pirates. They told me they were Mercs but they did also say privateers. I’ve mentioned to them a few times they need to change their acronym from a P to a B, and all they respond is “but then we wouldnt be according to plan and thats what we want to be called”. So regardless of what they told you, definitely not pirates. Probably closer to Lawless then, coz they want to steal do bounties, etc but certainly not pirate alliance. So I had no qualms about destroying one of their bases. They are KOS status for me as I’d like to be able to trust everyone with a P and as they said they’d take a bounty on me I clearly cant. Those flying a false flag should be hunted down and killed. Monin will be able to confirm.
To answer Fenra’s question, before last server wipe I worked with SWP (well they came and helped me). I personally would never attack anyone else with a P, and if they asked for my help I would if I could.
Feel like you should be putting a jailtime post on General Discussion then 
pirates attacking pirates is against the rules, so your claim isn’t without warrant.
No, Pirates can kill whoever, even other pirates. it’s not against he rules. There’s also no rule against pirates taking a bounty if they so choose. It’s just not their main forum on activity/income. 
A Pirate will take any opportunity that comes up. claiming a bounty on top of taking all their stuff sounds like a double win to me.
If some player or faction wishes to try and unite the pirates, you’ll probably have to do it with force, and incentive.