After trouble looking for sources to reference for my essay, I looked to interviews for an answer, and they did indeed yield some results, where I was able to find an interview with Miyazaki and some of his thoughts on Porco Rosso: http://www.angelfire.com/anime/NVOW/Interview1.html
I was able to find why he made Marco a pig, confirming my initial conjectures. I have also found small tidbits of information amongst review sites and blogs, that don't necessarily sway the argument, rather small facts that are there to consider and possibly give some aspects of the film context. I was also advised to consider a short excerpt from 'The Language of Animation', where the inherent innocence of animation is discussed. The topic gave me a nice basis for my essay, where I could discuss how Porco Rosso deals with the topic of representation, a large affair at the time of production, and how Porco Rosso possibly uses it's innocence to convey these affairs in a didactic form for people to consider in a more agreeable manner.