FPS Game Starter Kit uses the default UE4 skeleton, the only change is a GripPoint socket for where the character is holding the weapon. Any retargetting tutorials out there still apply. To see how to change the mesh click here.
The animations are set up so that they support IK for the left hand in order to fine-tune its position based on what kind of weapon the character is holding. The AnimBP reads the value from the weapon and adjusts the position, this makes it possible to reuse the same animations for multiple weapons.
If a weapon is drastically different than the others or if you have pre-made animations for all of your weapons, you can add them into the AnimCategory Enum and connect a different locomotion for each weapon.
FPS and TPS meshes use different AnimBP’s, both are based on the default UE4 skeleton, FPS template and TPS template respectively.
On a side note, the new optimization feature from 4.19 can be disabled if it’s causing issues on your PC.