how to connect 2 machines and share files and internet?

I have a 13 macbook pro retina laptop and with my tax return i will be buying a 2011 imac(not sure if it’ll be a 21 or 27 yet. it will depend on price and whats available at the time) i want a desktop for light gaming and some creative work that my laptop struggles to do so I really need a dedicated GPU. Now to the nuts and bolts of the issue. I am a full time student and live in a dorm so I need a way to physically share a single ethernet port between the two machine and actively share files between them without using the network to do it. i.e. i need these two computers to talk only to themselves when it comes to file sharing. Is this even possible with this combo of machines? I am not allowed to have a switch or router in my dorm room so i cant go the ethernet or lan route and target disk mode(if its even a protocol in the MBPr since it doesnt have a firewire port) isnt an option because there may be times where i need the internet from the imac to service the macbook pro(software updates and such) other then that the major concern is secure file sharing. Thanks guys.