Even more fun, the turrets for the two are the only difference, meaning you can simply pull out one turret and replace it with the other, depending on the mission. So your
mercenary unit only needs a strong crane to change out its firepower.
For other ideas, the vehicle's capacity is:
Body: 2 tons
Turret: 10 tons
(No heat Sink capacity or Power Amplifiers inherent)
So:
LRM-5 turret: 5*LRM-5, with 2 tons of ammo in the body
SRM-2 Turret: 10*SRM-2, with 2 tons of ammo in the body
SRM-4 Turret: 5*SRM-4, with 2 tons of ammo in the body (likely used the most as replacing the SRM-4s with LRM-5s is a simple 1:1 swap)
SRM-6 Turret: 3*SRM-6, with 2 tons of ammo in the body, and 1 MG with a half ton of ammo in the turret
3025-era Laser Turrets are not recommended, due to the Heat Sinks (and Power Amplifiers) needed. As an example, a single Large Laser would need 8 tons for Heat Sinks, 1 ton for Power Amplifiers, and itself weighs 5 tons for a total of 14 tons.
Not sure about Blazer turret options though.