For your Equipment Item Collection you'll want to change the available slots within the Item Slot Set defined inside your Item Slot Collection.
I think you'll want a single Slot called "Main Weapon" (or something similar) of Item Category "Weapon" or "Melee Weapon" or something similar.
Equipping armor and weapons is a must when talking about inventory and items. There are two types of equipment, skinned and not skinned. Skinned equipment . . .
opsive.com
Under the Item Slot Set section
The Inventory Item Set Manager simply monitors the Equippable Item Collections. What defines what can and cannot be equipped is defined by the Item Collections within the UIS Inventory. You can limit how items are added by using different Item Collection Types or using Item Restrictions.