My Spanish is bad I remember briefly watching some youtube video where a mexican was singing a corrido in Spain and said something like "yo soy un simple soldado".
Why is the adjective in front of soldado? Should it be "soldado simple"?
Spanish is a very flexible language. Though the normal use is "noun + adjective" we usually reverse the order. In the example you gave "yo soy un simple soldado" the adjective meaning is stressed by putting it first (I am just a soldier). If it were "Yo soy un soldado simple" the meaning would be slightly different (something like "I am a modest soldier").
In Spanish, sometimes we place the adjective before the noun (antepuesto) or after it (pospuesto). Sometimes it can change the meaning completely:
Una pobre viuda (a poor widow, I pity her but she could be rich or poor) // Una viuda pobre (a poor widow, she's got no money)
In your example, it would be: Un simple soldado (he's just a soldier and nothing more. He's not a general) // Un soldado simple (he's a plain soldier, not a sophisticated, not elegant etc)
In general, but not always, an adjective after the noun gives an objective meaning, whereas an adjective before the noun is subjective and gives a subjective meaning:
Alta cuna (high-born) // Cuna alta (a crib that is visibly high or tall)
I hope this helps.