First of all, if you bring in several different types of units from any modern army, you'll find the soldiers possess different weapons, uniforms, and equipment. Such variations by no means distinguish militia from regulars.

Secondly, we are told only a little bit of information about each group of soldiers that reaches Minas Tirith. What basis do we have for assuming that Duinhir's 500 archers are militia? None. How were they dressed? We don't know. Did they have officers? We don't know, though Duinhir and his two sons were obviously there leading them.

What about the 300 men from Ringlo Vale? How were they armed and armored? We don't know. We only know they had marched to war, had come many miles to reinforce Minas Tirith.

On the other hand Tolkien speaks of "a long line of men of many sorts, hunters and herdsmen and men of little villages, scantily equipped save for the household of Golasgil their lord" coming from Anfalas, and "fisherfolk from the Ethir, some hundred or more spared from the ships." Clearly these two groups are not regular soldiers. Tolkien tells us they are not.

But the others? They are distinguished from the militia by Tolkien's descriptions. Apparently they are soldiers sent out to reinforce the garrison at Minas Tirith.

Are they part of Gondor's standing army? We don't know.

Are they forces reassigned by feudal vassals to protect the Steward's city? We don't know.

Are they militia? Apparently not. They are not described as having other professions, being scantily equipped, or spared from other duties.

One must overlook several points Tolkien makes in order to conclude that all these guys were just militia.

: Now trying to get an accurate picture myself we see that
: Aragorn leaves MT with a force approximating 7 thousand (horse
: and foot).
: He also leaves Gondor in better defense then before he arrived.
: (keep in mind that he has also been reinforced by a force of 4
: thousand from Pelargir lead by Angbor by the time they march).
: Theodin brought less then 6 thousand spears (5 and Fifty
: Hundreds) to contribute to those totals above as well. Im sorry
: but I dont see armies numbering 40 plus thousand...

You're excluding the vast majority of the troops. I'm not surprised you don't see 40+ thousands. I'm not sure of how many were left after the battles at Ethring and Minas Tirith, either.

: ...Aragorn marches with a scant 7 thousand troops that are
: heavily reinforced by other regions. If Gondor had an army
: even close to "Estimation of the size of Gondorian armed forces
: is difficult but I would imagine it was between 40,000 and
: 50,000" (M.Martinez) they would surely be able to muster more
: then 7 thousand troops (REINFORCED) to march on Mordor.
: Now I realize that the army was to serve as a diversion for the
: ring bearers but they would also muster as many troops as they
: could spare for such an errand they werent suicidal just
: desperate.

But it was never Gandalf's purpose to lead a great army to Mordor.

  ...'I do not counsel you to leave the City all unmanned.
  Indeed the force that we lead east need not be great enough
  for any assault in earnest upon Mordor, so long as it be
  great enough to challenge battle.  And it must move soon.
  Therefore I ask the Captains: what force could we muster
  and lead out in two days' time at the latest?  And they
  must be hardy men that go willingly, knowing their peril.'

Aragorn's 6,000 Gondorians and 1,000 Rohirrim are not the last gasp of dying Gondor. They are just a token army intended to draw Sauron's forces out of Mordor, to clear the path for Frodo. All Gandalf hoped to accomplish was keep Sauron busy, and he took just enough soldiers to achieve this goal, while leaving Minas Tirith better defended than it was prior to the assault by the Lord of the Nazgul, in Aragorn's estimation.

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