A summary of the organizations of Armies in the 6th millenium.
Levies Units--untrained, mostly used for labor or diversions unless there is absolutely no alternative, as any better unit would chew them up badly. Primarly recruited/impressed Tech Level 2 peasants.
Militia Units--some training, capable of using simple firearms but not much else. Logistically supported using Tech Level 3 means--animal carts, trains, simple trucks at best. Used for internal security in Tech Level 2 and 3 areas and occasionally for battle, mostly as cannon fodder or relief for Line units. Armed with cast-offs from better troops, either bolt-action or single shot rifles and bayonets. Primarily recruited from kulaks (rich peasants) or townsmen. Some of these guys might be cavalry, although in RW armies, cavalry were always elite units.
Line Units--extensively trained troops armed with Tech Level 4 rifles, although probably not automatic rifles (see below). The main battle unit used in most conflicts, roughly equivalent to WWI or WWII level infantry. Some of these guys might be cavalry (the US Cavalry was disbanded in 1947 and saw mounted action in border conflicts with Mexico in the teens, and if oil or roads are rare cav is quite useful). Special units may make use of machine guns and simple armor, but this stuff would probably not be used heavily due to supply problems. Primarily recruited from Freemen or certain specially supervised and trained serf populations. The basic Muster unit would be of this quality.
Crack Units--These are the troops armed with higher tech level weapons, blaster rifles, shields, ceramsteel, hovertanks, aircraft, etc. The best Muster units here. This is probably the lowest Brother Battle unit. Noble amateurs would fall here.
Elite Units--Mobile Infantry in powered armor and drop ships, starfighter pilots, grimsons, dervishes, etc. Not too many of them and units would be quite small, but potentially decisive. Mostly recruited from "younger sons" or the upper echelons of the freemen who prove themselves in lower units. Rare but amazingly nasty when applied at the right time. A hundred Mobile Infantry are probably quite capable of completely routing literally thousands of Levies or Militia, and large units of Line troops would have an amazingly difficult time beating them at all. They are all pros, as maintaining this level of skill would require heavy training.
The latter two units types would probably only be used in unusual circumstances. Most Houses would be quite unwilling to commit their Elite Units where there is a risk of them being defeated.
Oh, I've noted that recruitment is class-biased. Officering would also be class-biased. Only nobles could be officers and commissions might be purchased or dispensed as favors. Probably it would also be a major public disgrace for a noble (if often privately a relief :) to be assigned to peasant Levies or Militia.
On the matter of automatic weapons, frankly a small shop style economy could not support this. The weapons could be manufactured that way--they're not _that_ complicated. Ammunition would be the real problem. Full auto guns chew through a truly enormous amount of rounds, the vast majority of which hit nothing at all. My guess is that it would be far more efficient, particularly if the troops using them are trained well and don't take high casualties (so the density of trained troops is always high), to use rifles like the Lee Enfield .303 bolt action used by the British during the first half of the 20th Century or the .30-06 M1 Garand used by the US from 1936 to 1959. Troops trained to use these rifles could, if needed, put up a substantial volume of highly accurate fire with far fewer rounds wasted, unlike troops using full auto rifles. Bolt action would be preferrable if ammo quality is not that high, as semiautomatic actions have trouble with iffy ammo, to say nothing of full automatic actions.