Depicting a single regiment would be problematic in terms of game scale. In general terms, a game on the scale of Global 1940 represents (admittedly in very abstract terms) the actions of units that are mostly the size of army groups and fleets – and, furthermore, of army groups and fleets which have no specific identity other than their nationality. It would be hard to justify saying that one particular regiment-sized unit, with a specific name, ought to be added to the game; by that rationale, there are hundreds of other regiments that could be added too.
The obvious exception to the above principle is the single fighter which represents the Flying Tiger squadron in China…so it’s not an ironclad rule that units smaller than army groups and fleets are never represented in the game. It can be argued, however, that the Flying Tigers are a special case because of their high profile, and because in the game China lacks aviation forces of its own. China doesn’t lack infantry, however, so adding a USMC unit doesn’t bring anything distinctive to China’s forces.
Additionally, Global 1940 has no rules that govern riverine operations – which is understandable, because the game map doesn’t depict any rivers. Even the Saint Lawrence, which is huge in real life, isn’t depicted: on the map, it looks like a strip of land. There are a few lakes depicted, plus a few canals and narrow straights, but no rivers. So a river patrol unit in China would literally have nowhere to go.