Is 4 mercs too many or is 8 to few?
How many calibers should you bring?
Do snipers get (or need?) match ammo?
How many machinegunners should you bring and is it a fixed number or is it a ratio like 1 per every 3 riflemen for example?
Should a combat squad have a dedicated medic or just a guy with a medkit?

For starters, I play on Expert or Insane. Highly recommend boosting starting cash as well as how much cash you get for selling to locals.
Additionally, this especially applies if you boost the Suppression numbers and/or play with AFS.
I usually don't fight at night, but I will move people around at night.

1) 4 is definitely too few, would recommend having at least 5 in each squad. What you do with that squad is up to you, but you keep those people together, breaking off only if, say, you're doing medic/militia training and can spare the additional manpower travelling with another squad.
2) If you're using calibers that the enemy uses, doesn't matter since you'll pick up more anyway. Other ammo like 6.8x43spc, keep an ammo crate in your backpack. Minimum of 4 extra magazines before you leave/enter any sector, keep extra ammo in your backpack.
3) Only if they don't have Marksman/Sniper traits, good stats, or a good rifle. Shoot it if you got it, but I don't recommend making it a purchasing priority.
4) Will explain my preferences later.
5) Everyone should have a small medkit, there should be at least one medic per squad. AND THAT MEDIC SHOULD SPOT, NOT BE INVOLVED WITH COMBAT.

I trained with the US Army IRL for a few years, they taught me the best way to travel is in squads of 10 or so. For JA2's sake, that'd mean having two squads of 4-6 traveling together at all times. For simplicity's sake, lets call them Alpha and Bravo team.

Both fireteams have:

1 AutoGunner (Has Skill and/or is specialized in it, i.e. MachineGunner)
1 Designated Marksman (Does not have to have Marksman/Sniper Skill, but if he/she doesnt, must have high Marksmanship + Dexterity)
1 Grenadier (Heavy Weapons) or Demolitions (optionally replace grenade launcher with thrown grenades if merc has Demolitions)
1+ Riflemen (Serve varying roles)

Between the two fireteams traveling together, there should ideally be:
1 guy carrying a GPMG/HMG (7.62x51/x54R or bigger), NOT one of the auto gunners above
1+ Medic. Paramedic/Doctor Skill required.
2+ Spotters. Must have Spotter Skill for at one of them at minimum. This means Ira or Raven or an IMP. Other one doesnt need one, but they both need binoculars.
1+ guy with Demolitions regardless of if he takes the "grenadier" role.
Ideally, 1+ people with Deputy/Squadleader. Keep him within 10 square proximity of other soldiers. He is an ideal candidate to additionally serve a spotting role.

As for weaponry:
Auto Gunners: Has a LMG. 5.56/5.45/7.62x39 or similar caliber. One of these guys has a GPMG/HMG.
Marksmen: Semi Auto precision rifle of any kind. You're smart, you should find something good.
Grenadiers: Assault rifle + Grenade launcher that fits the rifle.
Riflemen: Assault rifles, or Battle rifles if you can afford to manage the extra ammunition use. SMGs are okay for suppression.
Spotters: Needs binoculars for every spotter. Recommend giving them assault/battle rifles instead of snipers, though.




How to operate operationally on your operations:

Alpha team leads, especially with a spotter (binoculars).
Spotter locates enemies.
Alpha team hides behind cover, sets up out of visual range (30+ tiles if possible)
Bravo team moves to flank from another angle, preferably at a 90deg angle from where Alpha team is aiming at enemies. Optionally, have them at an ambush location behind a building. If you do this, have someone be a rear guard in case they come around the back.
Someone from Alpha team shoots.
Enemies come running.
Bullets fly.
Keep Bravo team alert, as enemy might flank.
???
Perform general sector sweeping to eliminate stragglers
Profit!