Is there a way to create my own custom weapons for like one of my custom capital ships.
you can create custom weapons in projectiles.xml. You can use any of the effects that are in use for the game for weapons and even change the appearance of turbolasers and ion cannons. You can chnage the size, color and even the shape of the lasers, turbolasers and ion cannons.
But can I like create like my own type of weapon. like a weapon that could destroy lets say a Mon Cal Crusier in one hit.
Yes you can, but you need to work on it very hard, because even Avenger who made Death Star shooting cruisers didn't make it to kill all of them 100% in 1 hit.
is it possible to add like a plasma laser or other guns that you might think of your self. Don't you have to create your own projetile file.
Yes, you just make new projectile and customize everything in it.