Hello Josh! From my point of view, it is really good that you try to learn Spanish grammar so specifically, but it is not very helpful in many cases. I recommend you to try learning without trying to 100% understand how Spanish grammar works, because as in English, Spanish has fixed phrases or fixed verbal phrases that may not make much sense not even for Spanish native speakers, but they were made so by somebody and it stayed as an universal rule for Spanish.
In order to answer your questions, I'm going to explain how they grammatically work and why your thought about them is a little bit different from the correct thought you should have about them.
First sentence: "Así que corre corre corre corazón, de los dos tu siempre fuiste el mas veloz."
This famous sentence from the song "Corre Corazón" is grammatically correct. But you may wonder, "why?". Well, it's simpler than what it seems to be.
"Fuiste" is a synonym of "Has sido", and as you may know, "has sido" works grammatically well with the time adverb "siempre". So, if you replace "tu siempre has sido" by "tu siempre fuiste" (because "fuiste" is "has sido" 's synonym), it would work the same way.
To sum up, "tu siempre has sido" (you have always been) and "tu siempre fuiste" (you have always been) are the same.
I hope you do not confuse "fuiste" with "eras". "Eras" in English means " were" (because it's conjugued for the second person of the singular, "you" in english and "tu" in Spanish). So, "fuiste= has sido = you have been" and "eras = you were".