pretty sure you can join w/e you want. Just need a high enough rep with them

You need a high enough reputation level, from there you take a trial against three of their knights of whatever specific order you're joining. For forming a KO, you need a specific level of honor. Just read this. https://pop3.fandom.com/wiki/Knighthood_Orders

Would i be able to join a KO then form one? Or is it only one or the other? In my campaign that's what im struggling with. Do i want to join one or make one? That is my dilemma.

KO and CKO (custom, player made) are not mutually exclusive - you can enjoy both at the same time.
Join any KO you like, do some quest for them to be promoted to given KO grandmaster (and get some benefits).
Create, but not join, you own CKO - because you started it, you are automatically their honorary grandmaster. If you decide to join them, e.g. for the role-playing reasons, you can, but you will loose access to some features.

So I got all that down. Thank you!! Now a simple question. Which knight order should i join for foot soldiers and archers?

I recommend Ebony Fist or Griffon Knights. EF gives you prisoner-taking sergeants and the knights are siege monsters. Truly ideal sieging knights; heavy crossbows with the health, armor and weapons to swap to assault when the conditions call for it.

Griffon Knights are excellent field-battle troops; both the sergeants and knights. Their creation and joining is tricky. Check the wiki on how. They also eventually give you an amazing unique armor that is, when upgraded, only exceeded by the rune plates.

If neither of those sound up your alley, I've completed a run as the Shadow Legion GM and they are also extremely powerful.

as for joining, you should join the one that gives you the best equipment (or coolest looking) for your build.

speaking of the chapters you can create in your fiefs, the best KO infantry are the krakens and the best missile knights are silvermist rangers. there isn't really a KO that combines both archers and decent foot soldiers (shields are pretty much necessary for the very op nature of missile troops in vanilla pendor).

to conclude, the best cavalry KO is (by far) the shadow legion.