Must I use "fèk" and "sòt" together? Can I say "Nou fèk wè yo" and "Nou sòt wè yo", or does it need to be "Nou fèk sòt wè yo"?

Fèk and fèk sòt indicates a more immediate past.

Yes, you may use fèk all by itself.

Nou fèk wè yo  (We just saw them) is accurate.

And yes, you will hear Creole speakers use "sòt" by itself.

sòt (sòti) → to come from

Nou sòt wè yo (We come from seeing them)

And, Nou fèk sòt wè yo (We just come from seeing them) is also accurate.

So, you're good with all three :)

Ask me anything

is "fek" translated as "just" in English?

Yes.

Just (as in a moment ago) → fèk, fèk sot

Check this link out:
http://sweetcoconuts.blogspot.com/2011/08/how-to-translate-just-in-you-just-dont.html

Ask me anything

how to translate 'just" in "You just don't understand"?

Just (merely, simply, nothing but) - jis  or  sèlman

You just don't understand.

Ou jis pa konprann.

I just want to talk to you.
Mwen jis vle pale avè w.

We are just friend.
Nou jis zanmi.

Just (recently, a moment ago, at this moment) - fèk  or fèk sot

We just arrived.
Nou fèk rive.

I just fixed it.

Mwen fèk repare li.

I just saw her.
Mwen fèk sot wè li.

Just (absolutely, perfectly, exactly) - egzakteman

This is just what I was looking for.
Sa se egzakteman sa mwen t'ap chache a.

This house is just like my friend's house.

Kay sa egzakteman tankou kay zanmi mwen an.

You are just like your dad.
Ou egzakteman menm jan ak papa w.