Fui or soy.....

Fui or soy.....
 

se pensó una otra chica que fui de Venezuela
what does this say? is it well written?

If you're trying to say "Another girl thought I was from Venezuela," that is said "Pensó otra chica que (yo) era de Venezuela."

Since the subject is in the sentence, there's no need for the impersonal 'se'.
another = otra
You use the imperfect tense instead of the preterite to express ser de in the past (it didn't have a start and an end in the past, but was an ongoing condition).