From a very quick look, it seems the USMC had 1st, 2nd, 4th, 5th & 6th Infantry Regiments and 10th Artillery Regiments active on 1st Jan 1936. From what I can tell they were all under strength, just as the regular army units were at that time. Best representation may be as a 3 or 4 units using the garrison template with 3 x Inf. One of the good things with the upcoming patch/DLC will be the ability to combine units, meaning that these units could be combined into a larger Marine template later on after its been researched. I note that in terms of the development of landing craft, the British started in WWI with the Japanese following in 1937 and the US after that, so there's probably enough justification to represent them as infantry rather than marines in 1936.
With the addition of the marine corps, and mechanised cavalry, at game start the US should probably lose all those pacific garrison units.