In terms of MGs, by the War's end every British Infantry Division had 64 Vickers Machine Guns (Canadian divisions had 96) and 36 Lewis Guns per Infantry Battalion (that means 324 per British/ANZAC division, 432 per Canadian Division), with 108 Hotchkiss Mk. III (Benet-Mercie Mle 1909) Light Machine Guns per Cavalry Division. French Infantry Divisions had 108 Hotchkiss M1914/St. Etienne M1907 Machine Guns and 3 Chauchat Automatic Rifles per platoon, for a total of c. 324 per division. The Germans had similarly increased the amount of MG-08 and MG-08/15 Machine Guns per infantry division, as well as creating Machine Gun Sharpshooter Battalions that could be attached to Corps headquarters and assigned to sections of the front or to divisions as needed.
However, of these weapons only the Chauchat was really light enough for 'walking-fire.' This was technically possible with the Lewis Gun, but this required the use to stop before firing and was very inaccurate. The MG-08/15 was basically an MG-08 with a pistol grip and trigger, rifle butt and bipod. The bipod was very unstable, and the weapon itself weighed about as much as the British Vickers Gun. Walking-fire was very difficult, being pretty much unaimed and taxing to the user physically. The Hotchkiss Mk. III, like the MG-08/15, was able to be moved around on the battlefield more quickly than heavier MGs, but was to be fired stationary, and it's 'strip' magazines required a smooth feeding action to avoid jams. Since all these weapons save the Chauchat are in the game and can be carried much as one would a SAW or L86 LSW in modern warfare FPS's, this is certainly 'artistic license' on the part of DICE.
As for SMGs, particularly the MP-18 and Beretta M1918, these entered service at the same time, in early autumn 1918, and only a few thousand were made. Probably still less saw actual combat, and so their widespread availability in the game would, again, be 'artistic license.'
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