Okay, ran some numbers today and did some play tests. Missile turrets are clearly the favored turret to use but why? Because they are the only small turret that isn't broken.

Small missile turret:
Direct damage 455, splash 357.5 over 2.5 meters. Clip size 8
129% effective vs armor, 591.5 per hit (4,695.6 damage per clip)
69% effective vs shields, 314 per hit (2,778.6 damage per clip)
Splash damage is almost equal to a direct hit from a railgun.
Easy to hit infantry from an elevated position.

True damage: As listed but is +29% vs armor, -31% vs shields

Small railgun turret:
Direct damage 377, clip size 24
63% effective vs armor, 237.5 per hit (6,604.3 but overheats at half clip so really is 3,300)
77% effective vs shields, 290 per hit (7,356 but overheats at half clip so really is 3,676)
Almost impossible to hit infantry from an unstable platform.

Ture damage: 263.9 (+7% vs shields, -7% vs armor)

Small blaster turret:
Direct damage 49.4, clip size 150
33% effective vs armor, 16 per hit (2,445 but overheats at half clip so really is 1,222)
44% effective vs shields, 21.7 per hit (2,714 but overheats at half clip so really is 1,357)
Almost impossible to hit infantry from an unstable platform.

True damage: 19 (+8.5% vs shields, -8.5% vs armor)

What is up with these numbers? Your closest range turret (highest risk) has the poorest DPS while missiles are effective out to 250 meters (railguns only go 300) with splash damage almost equal to a direct railgun hit? Then you give railguns and blasters an overheat mechanic that effectively halves their clip size because the overheat duration is almost as long as reloading.
The blaster has such poor DPS that a mass driver and flux grenades are more effective vs tanks/installations. Coupled with the fact that it is almost impossible to hit infantry with it on an unstable platform this turret is literally useless. Period.

The efficiency ratings on hybrid weapons are just messed up but missiles get their actual listed values (+30% vs armor, -30% vs shields). If you were worried about railguns and blasters being overpowered then lower their damage, don't leave these messed up efficiency ratings. You are

Not all around, I'd say, but maybe 120 degrees forward. That way the gunner can kill what he's shooting at, but flanks will still be effective against him.

Yes, but the gunner should be facing the enemy, protecting both him and the driver.

I was talking about armor that's mounted on the turret, so it'd rotate with the gunner.