Let's assume, for the sake of argument, that the Federation decided to revive the idea of the MACOS from Enterprise, and went into a full recruitment and deployment rollout, including revamping the concepts of personal armor and troop transports/shipboard compliments.
1. How would you actually go about recruiting soldiers, (must be volunteer only)?
Bajor. First and foremost I recruit Bajorans to lead this army and design the training scheme. I would be tempted to headquarter the Federation army on Bajor, and certainly give the top jobs in training to ranking personnel in the Bajoran militia. Compared to every other Federation member they have a recent history of ground combat. Secondly, they could use the prestige. Bajor isn't liable to be contributing heavily to starfleet any time soon, thanks to the relatively low technology environment there, until its education system is fully integrated, but they know everything worth knowing about ground combat. Obviously it'd not be restricted to Bajor or anything so savage as that, but that's where the expertise is, and it's also likely to be where a lot of recruits would come from.
Assuming relations with our allies (Klingons, etc) are good, I'd also arrange for war games and such with them. I'd also hire any Hirogen, etc I can get on the payroll to provide training.
I'd have recruitment follow starfleet lines, but obviously with a more military focus. Perhaps use the occupation of Betazed to highlight the patriotic need to have a strong army.
2. How you you equip them, (can only use technology demonstrated in the various media, no 1-off superweapon stuff).
Lightweight Body Armour, a-la the Klingons (there's a few episodes where it demonstrates resilience to phasers on stun and light kill) for which I will probably hire klingons. I would look to upgrade this to fully-sealed armour to provide NBC protection, and provide them with grenades, rifles and so on.
If we're taking the new films as illustrative of technical canon, I'd look to give them equipment capable of orbital drops as in the ST reboot, if my experts say there's a worthwhile advantage to that.
If possible, each soldier will have a backpack that carries a transporter inhibiter that they can use, for obvious reasons, and every man will carry a transport enhancer for the same reasons. These will have to be ruggedized for field use.
Design priorities are Machine Guns, Mortars (as in the old days), Artillery, IFVs, tanks, aerospace craft (Can I get the Son'a/Bak'u to sell me the design for theirs? Those are great.)
3. What vessels would you have retrofitted or build additional, (again, keeping within established and fully realized technology)?
I buy B'rels. Those things can beam up a hundred tonnes at once or something.
More seriously, I look at creating a troopship pod for the Nebula class; and quartering arrangements for older ships like Excelsiors and Mirandas, of which there seems to be an inexhustible supply.
4. Where would you station said soldiers, (keep a contingent on board all vessels, keep them in reserve to deployed only when needed, etc)?
I give Starfleet Security army equipment, but keep the troops seperate. We do not need our defensive army going out on five year missions, nor do we need to be denying starfleet ships mission specialists they could carry otherwise. If there's a need for them on ships in defensive patrols, obviously they can be added, and they'd be deployed in wartime.
If Sela was right, and two thousand soldiers was enough to cause Vulcan to surrender, then obviously Vulcan is a clear priority for a garrison, and that's going to be their first job.
5. How or where would you train said soldiers?
Large holodecks; assuming it's viable, the holoship, and I think the Hirogen(?) uses of it in Voyager demonstrate that a holosuite can be built to house hundreds of people in one simulation, allowing for entire companies to train at once; other than this, training would of course also be in the real world at times, and I'd look to have them trained in varying climates, gravity and atmospheres, as required.
6. How would the chain of command function, (reports to ship's captain, has separate directives, give them their own vessels to operate)?
Indepenendnt rank structure, reporting to the Federation Council, or whoever Starfleet Command reports to. Transport would be provided by starfleet vessels as required however, to allay domestic concerns about the military as much as possible. Starfleet should remain the 'senior service' and the military should support starfleet when necessary, but be more limited in its policies.
7. Any other significant points you'd address or policies you'd enact? Would you further break this force down into more specialized squads/corps, perhaps?
Some species would obviously have their own units if too divergent from the progenitor/humanoid baseline. I'm looking at you, Horta.
I'd look to have a specially trained Betazoid unit, these would be used in occupation duties, if we have to do that, to assist at checkpoints and otherwise sense hostile intent. Seperate from this we will obviously make full use of the Federation's advanced mental health care to protect soldiers' mental health.
Additionally, and somewhat violating the word if not the spirit of the 'no one off superweapons' I would make development of a viable hovering autonomous drone approximately one to two feet tall a top priority. We've seen that this is fiesable, with Echo-Pappa 206 in
Arsenal of Freedom and the Son'a capture drones in
Insurrection. Something along these lines is such an advantage in urban combat and other environments that we'd be frankly mad not to use them if it's at all cost effective. Both examples come from cultures with a higher level of technology in at least some areas though, and there may be treaty restrictions against fully automatic kill-bots, but it's something we must have if we can. The son'a transporter tags are a good idea for a less lethal weapon too, and I'd look into getting a rifle version made for any scenarios where stun doesn't work.
Theatre shields are another top priority for development. We know that a planetary shield is
physically possible from a few one-off examples, and we need to work on that. Along with those we need to work on planetary defences, we know they're viable from the fact that they were used to attack the borg ship in BoBW; but clearly we need to see what can be done to beef those up.
I approach the Klingons and Romulans, if at all friendly (such as, say, the Romulan Republic in STO), about developing interoperable equipment. If politically fiesable, I also arrange people to visit the Cardassians, as their ground equipment is reportedly impressive, and includes armoured vehicles. Our objective of course should be to make the Cardassians our friends, though obviously this may be doubly complicated as I mean to heavily recruit Bajora. We probably have captured cardassian gear from the war anyway.