interesting observations I've noticed, based on personal anecdotes so I could be wrong, but they are interesting nonetheless. =p
delta istps will mistake infps as being in their quadra at first, will mistake the infps lovey dovey ness for something that is Fi-valuing and will be encouraging and uplifting to them, but then once it's clear that they are infps the conversation turns awkward and the istp will get suspicious/angsty towards the infp and the infp will stop idealizing the istp, and kind of feel repulsed that they're so hostile and suspicious with them.
duals do ignore each other first for a long time, it's like you live with somebody or be around somebody so LONG and then like a ton of bricks, something happens and that person is the only one having your back and it's like 'I thought you were so boring and just there but this is twu luv!
contraries are very superficially chummy, and gossip easily about stuff. But don't EVER take it too far or talk about what you REALLY believe in with each other, that's a recipe for annoyance.
intps like the infps too, like I said at first glance but they roll their eyes @ the infp's idealism and puppy dog loveness. I would say though this relationship is very good for discussing politics and 'wordly' things but when you get close and personal, it's too tense and awkward.
illusariones understand you very well personally but can't do anything to help you or move you in the direction you need to go. Pure understanding is so overrated. You may think 'ALL I NEED IS SOMEBODY TO UNDERSTAND ME!' *emo tear* but no bitch, yeah you need that but you also need A LOT more than that, and it's something ultimately only your dual can provide. My relationship with steve and eldanen is SO much like this. And it makes you realize that 'okay people understand me, but now what?' It's nice but limiting.
Identicals really aren't as neutral as people say, they are kind of like this 'calm water' with maybe this interesting storm that is constantly brewing underneath the surface but never totally unleashes. However, like your dual, when push comes to shove your Identical will defend you when you really need it. Also your Identical kinda sees through your ego which is nice, and in real life they do tend to activate you more than you think. It's just the main point I'm making is when something really matters, your Identical is there for you but they won't really have your back for a lot of the superficial every-day drama that you need to vent, other types are just more suited for that.
Activity and Mirror are nice but too ego-based, there's really not enough touch, like they don't get to you- you just have the same ideals, world views, and egoic delusions and fantasies, but they don't cut through the veil and just SEE YOU as your dual and identical does. But for a business relationship I'd say they're maybe better.
I'll have more later.