I suppose that defensive turrets are not really necessary for ships of this size?
More of velocity, not size.
It's a decent option to have kind of slowish patrol corvette or frigate - and it's quite reasonable to arm them with turretted multirole cannons (like 10-cm QF lasers). They'll be able to weather missile salvos of some lone light missile ship, will have some chances to shear off all armour of light beam combatants, and if they'll encounter unarmored merchants or surveyors - they'll rip them apart without expending their ammo.
I guess both the missiles and gauss cannon (if the defensive armament won't be necessary) can be swapped out for either a laser cannon or a plasma carronade.
The problem of specialized defensive (anti-missile) armament for such light ships is not about necessity, it's about sufficiency. If your ship is armoured and also have decent anti-ship armament - she'll have not enough volume to mount anti-missile weapons capable of defending her against missile opponent of her size and tech level. She will be dead in encounter with missile-only opponent and she will be dead in encounter with beam-only opponent (because the latter will be heavier armed for decisive close combat). Mixed armament is more resonable choise for larger ships (smth like large cruiser with raider specialization), because there will be decent probability of encounter with smaller opponents and she will have an option to choose which weapon to use in this case to reserve her armour or her ammo according to the situation. Corvette just will not meet smth smaller and lonely, because there is no smaller long-range combat ship and no wild FAC will travel lonely outside of her base's missile range (and even decent FAC will rip your corvette apart with missile salvo, by the way).
Speaking of plasma carronades - they are very capable for defending jump points and bombarding defenseless worlds, but not for any other role. They are just too shorthanded.
Isn't ECCM supposed to be per fire control? Last I checked fleet fire controls, there was an option to select eccm for a fire control.
While ECM - it is not yet developed.
Sorry, my misreading.
Really, it's absolutely reasonable to have several ECCMs per main combat ship, though it's not mandatory to have ECCM for
every FC (for example, I'll have no additional ECCM for spare short-range FC that is added just for redundancy at some corvete or heavy FAC).