For the Campaign, you can modify the milestone_114_sim_leopard_start.json to change what is in your starting parts room. Is there a way to mod the starting weapons & equipment inventory in career mode?
Yes you can mod the start file (make a back up first of the original file) for example
Item.WeaponDef.Weapon_Laser_MediumLaser_0-STOCK",
"value" : 2
can be changed to
Item.WeaponDef.Weapon_Gauss_Gauss_0-STOCK",
"value" : 2
change the value to what you want