to put it simple Fortuna Kitguns have a stat like this
Gaze : Fire rate ++ | Damage - -
Rattleguts : Fire rate + | Damage -
Tombfinger : Fire rate - | Damage +
Catchmoon : Fire rate - - | Damage ++

[Gaze] is just lazer beam have punch through and chain i don't recommend to play it or invest in it at all. you better invest on [Kuva Nukor] or [Tenet Cycron]. combo with Pax Charge due to high fire rate, not good with Pax [Seeker] due to low damage
[Rattleguts] is just a compact uzi. combo with Pax Charge due to high fire rate, not good with Pax [Seeker] due to low damage
[Tombfinger] is revolver or high caliber pistol like desert eagle. quiet fun to play. combo with Pax [Seeker] due to high damage
[Catchmoon] is a handcannon low range bad damage falloff high damage very satisfy when you saw those big number of damage. combo with Pax [Seeker] due to high damage

[Tombfinger] > [Catchmoon] = [Rattleguts] > [Gaze]

Exilus slot is just personal preference. feel free to leave it empty