If I am correct in assuming that there is a difference between being one of the proletariat and being a plebeian, in that a pleb, in exchange for potential military service, is given the right to participate in governing but the proletariat are not, how did the military reforms (specifically, recruiting proletariate into a newly professional military) Gaius Marius is accredited of instituting affect the distinction between the two classes from the time of the Republic onward?
To address the first part of your question, your assumption is basically correct, although your terminology is rather muddled. I'll clear this up, as understanding the various terms in use is important to having a proper handle on your reading, after all.
Plebians and patricians were families who were respectively not traceable and traceable back to the beginning of the Republic. By Marian's time, the distinction had ceased to have a serious economic impact and couldn't be said to be indicative of class, which I'm assuming is part of your understanding given your use of Marxist terminology.
The pre-marian Roman military didn't make a distinction between being a plebian or a patrician, as long as you were sufficiently wealthy to equip yourself. In this sense, "plebians" were not necessarily proletarian or non-proletarian, because by Marian's reforms the legal distinction between plebians and patricians was effectively nominal, which is to say they were of little practical consequence. A poor plebian would be what you would call "proletarian", and a rich plebian would not. Same goes for the patricians.
Hope this helps. I recommend Mary Beard's popular history SPQR for further introductory reading. It is quite accessible, and her biases shine through sufficiently that it is easy to read around them.