Your ship has a bunch of stats, and the following are “limitations” that control what you can fit:
High Slots - missiles and guns of various kinds require high slots
Turrets vs Missiles slots - into High Slots that the ship has, you can only put so many guns or missile launchers
Drone Bay - drones
Power Grid - everything requires megawatts (MW) of power from the generator, you cannot install guns, missiles, or other modules that are too big and consume more MW than your ship has. The Power Grid of the ship ensures that you install frigate-sized stuff into frigates, cruiser-sized stuff into cruisers, and so on.
CPU - everything also requires some processing power (teraflops - trillion floating-point operations/calculations per second - TF). Industrial or electronic jamming modules require a lot more CPU teraflops than guns, armor, and shields, so the CPU usually limits you from installing industrial or electronic stuff (mining lasers, probes, cloaking devices, electronic jammers) on “combat” ships that aren’t really intended to have that stuff.
So, as above, the Merlin has 3 high slots, but is designed for 3 turrets 0 missiles, it’s a gun ship. You can put any of the guns in it (Minmatar autocannons or artillery, Caldari/Gallente railguns or blasters, Amarr lasers), but the ship has bonuses for “hybrids” = rail guns or blasters, so you’ll do more damage if you use the Caldari guns.
Kestrel has 4 high slots, and also 0 turrets 4 missiles so it’s a missile ship. The ship bonuses and the limited Power Grid will force you to use either rockets (short range) or light missiles (longer range) on the Kestrel. In order to fit the entire complement of 4 Light Missile Launchers you may need to increase the Kestrel’s Power Grid by installing a Micro Auxiliary Power Core in one of the low slots.
Also, there are very few skills that increase Power Grid and CPU for ships, so it’s recommended that you train these few skills to 4 or even 5. Look in the Engineering group of skills in your character sheet, and train the Power Grid and CPU / Processor skills there; they’ll serve you well, with all your ships in the future.