How do you know where to place an adjective in a Spanish sentence?

I know that, in Spanish, an adjective normally goes after the verb it describes, for example, "los ninos feos" or "los gatitos bonitos." But I have also seen people use "feos ninos" and "bonitos gatos" in a sentence. Does that change the meaning of the adjective when you do that or does it not matter? Also, I looked up "bad breath" in Spanish, and it seems that it is said "mal aliento." Why would it not be "aliento mal?" Also, I have heard the sentence, "es de mala educacion a ponerte los codos en la mesa." Why is it not "educacion mala?" Thank you to anyone who can answer.