In the Journals of the Lewis and Clark expedition of 1805 a notation on the diet of local Indians says, "They live on elk, deer, fowl, but principally fish and roots of three kinds." If you don't like that one I have over 43,000 other references, many in the biological sciences.
principalmente is an adverb, not an adjective. As for adjectives, the answer is "it depends". Some types of adjectives, such as numbers, are placed before the noun (uno perro, tres gatos). Most go after the noun, but some can be placed either before or after it.
This article has more information: http://spanish.about.com/cs/grammar/a/whereadjective.htm