Fui or soy.....

[hola]

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

[Rusty]

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).